mdadm test "10ddf-create-fail-rebuild" triggers warnings like the following

[  215.526357] ------------[ cut here ]------------
[  215.527243] WARNING: CPU: 18 PID: 1264 at block/bdev.c:617 blkdev_put+0x269/0x350
[  215.528334] Modules linked in:
[  215.528806] CPU: 18 PID: 1264 Comm: mdmon Not tainted 6.4.0-rc2+ #768
[  215.529863] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
[  215.531464] RIP: 0010:blkdev_put+0x269/0x350
[  215.532167] Code: ff ff 49 8d 7d 10 e8 56 bf b8 ff 4d 8b 65 10 49 8d bc
24 58 05 00 00 e8 05 be b8 ff 41 83 ac 24 58 05 00 00 01 e9 44 ff ff ff
<0f> 0b e9 52 fe ff ff 0f 0b e9 6b fe ff ff1
[  215.534780] RSP: 0018:ffffc900040bfbf0 EFLAGS: 00010283
[  215.535635] RAX: ffff888174001000 RBX: ffff88810b1c3b00 RCX: ffffffff819a4061
[  215.536645] RDX: dffffc0000000000 RSI: dffffc0000000000 RDI: ffff88810b1c3ba0
[  215.537657] RBP: ffff88810dbde800 R08: fffffbfff0fca983 R09: fffffbfff0fca983
[  215.538674] R10: ffffc900040bfbf0 R11: fffffbfff0fca982 R12: ffff88810b1c3b38
[  215.539687] R13: ffff88810b1c3b10 R14: ffff88810dbdecb8 R15: ffff88810b1c3b00
[  215.540833] FS:  00007f2aabdff700(0000) GS:ffff888dfb400000(0000) knlGS:0000000000000000
[  215.541961] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  215.542775] CR2: 00007fa19a85d934 CR3: 000000010c076006 CR4: 0000000000370ee0
[  215.543814] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  215.544840] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  215.545885] Call Trace:
[  215.546257]  <TASK>
[  215.546608]  export_rdev.isra.63+0x71/0xe0
[  215.547338]  mddev_unlock+0x1b1/0x2d0
[  215.547898]  array_state_store+0x28d/0x450
[  215.548519]  md_attr_store+0xd7/0x150
[  215.549059]  ? __pfx_sysfs_kf_write+0x10/0x10
[  215.549702]  kernfs_fop_write_iter+0x1b9/0x260
[  215.550351]  vfs_write+0x491/0x760
[  215.550863]  ? __pfx_vfs_write+0x10/0x10
[  215.551445]  ? __fget_files+0x156/0x230
[  215.552053]  ksys_write+0xc0/0x160
[  215.552570]  ? __pfx_ksys_write+0x10/0x10
[  215.553141]  ? ktime_get_coarse_real_ts64+0xec/0x100
[  215.553878]  do_syscall_64+0x3a/0x90
[  215.554403]  entry_SYSCALL_64_after_hwframe+0x72/0xdc
[  215.555125] RIP: 0033:0x7f2aade11847
[  215.555696] Code: c3 66 90 41 54 49 89 d4 55 48 89 f5 53 89 fb 48 83 ec
10 e8 1b fd ff ff 4c 89 e2 48 89 ee 89 df 41 89 c0 b8 01 00 00 00 0f 05
<48> 3d 00 f0 ff ff 77 35 44 89 c7 48 89 448
[  215.558398] RSP: 002b:00007f2aabdfeba0 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
[  215.559516] RAX: ffffffffffffffda RBX: 0000000000000010 RCX: 00007f2aade11847
[  215.560515] RDX: 0000000000000005 RSI: 0000000000438b8b RDI: 0000000000000010
[  215.561512] RBP: 0000000000438b8b R08: 0000000000000000 R09: 00007f2aaecf0060
[  215.562511] R10: 000000000e3ba40b R11: 0000000000000293 R12: 0000000000000005
[  215.563647] R13: 0000000000000000 R14: 0000000000000001 R15: 0000000000c70750
[  215.564693]  </TASK>
[  215.565029] irq event stamp: 15979
[  215.565584] hardirqs last  enabled at (15991): [<ffffffff811a7432>] __up_console_sem+0x52/0x60
[  215.566806] hardirqs last disabled at (16000): [<ffffffff811a7417>] __up_console_sem+0x37/0x60
[  215.568022] softirqs last  enabled at (15716): [<ffffffff8277a2db>] __do_softirq+0x3eb/0x531
[  215.569239] softirqs last disabled at (15711): [<ffffffff810d8f45>] irq_exit_rcu+0x115/0x160
[  215.570434] ---[ end trace 0000000000000000 ]---

This means export_rdev() calls blkdev_put with a different holder than the
one used by blkdev_get_by_dev(). This is because mddev->major_version == -2
is not a good check for external metadata. Fix this by using
mddev->external instead.

Also, do not clear mddev->external in md_clean(), as the flag might be used
later in export_rdev().

Fixes: 2736e8ee ("block: use the holder as indication for exclusive opens")
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Song Liu <song@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20230617052405.305871-1-song@kernel.org