btrfs_free_extra_devids() updates fs_devices::latest_bdev to point to
the bdev with greatest device::generation number.  For a typical-missing
device the generation number is zero so fs_devices::latest_bdev will
never point to it.

But if the missing device is due to alienation [1], then
device::generation is not zero and if it is greater or equal to the rest
of device  generations in the list, then fs_devices::latest_bdev ends up
pointing to the missing device and reports the error like [2].

[1] We maintain devices of a fsid (as in fs_device::fsid) in the
fs_devices::devices list, a device is considered as an alien device
if its fsid does not match with the fs_device::fsid

Consider a working filesystem with raid1:

  $ mkfs.btrfs -f -d raid1 -m raid1 /dev/sda /dev/sdb
  $ mount /dev/sda /mnt-raid1
  $ umount /mnt-raid1

While mnt-raid1 was unmounted the user force-adds one of its devices to
another btrfs filesystem:

  $ mkfs.btrfs -f /dev/sdc
  $ mount /dev/sdc /mnt-single
  $ btrfs dev add -f /dev/sda /mnt-single

Now the original mnt-raid1 fails to mount in degraded mode, because
fs_devices::latest_bdev is pointing to the alien device.

  $ mount -o degraded /dev/sdb /mnt-raid1

[2]
mount: wrong fs type, bad option, bad superblock on /dev/sdb,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.

  kernel: BTRFS warning (device sdb): devid 1 uuid 072a0192-675b-4d5a-8640-a5cf2b2c704d is missing
  kernel: BTRFS error (device sdb): failed to read devices
  kernel: BTRFS error (device sdb): open_ctree failed

Fix the root cause by checking if the device is not missing before it
can be considered for the fs_devices::latest_bdev.

CC: stable@vger.kernel.org # 4.19+
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>