drm/i915: Add mmap lock around vma_lookup() in the mman selftest.

Add mmap_read_lock/unlock around vma_lookup(). The core code requires
this for lookups. Since we only check if the return value is NULL,
we can immediately unlock.

This fixes the following splat in the selftes:

i915: Running i915_gem_mman_live_selftests/igt_mmap
------------[ cut here ]------------
WARNING: CPU: 3 PID: 5654 at include/linux/mmap_lock.h:164 find_vma+0x4e/0xb0
Modules linked in: i915(+) vgem fuse snd_hda_codec_hdmi snd_hda_codec_realtek snd_hda_codec_generic ledtrig_audio mei_hdcp x86_pkg_temp_thermal coretemp crct10dif_pclmul crc32_pclmul ghash_clmulni_intel snd_intel_dspcfg snd_hda_codec snd_hwdep e1000e snd_hda_core ptp snd_pcm ttm mei_me pps_core i2c_i801 prime_numbers i2c_smbus mei [last unloaded: i915]
CPU: 3 PID: 5654 Comm: i915_selftest Tainted: G     U            5.15.0-rc1-CI-Trybot_7984+ #1
Hardware name: Micro-Star International Co., Ltd. MS-7B54/Z370M MORTAR (MS-7B54), BIOS 1.00 10/31/2017
RIP: 0010:find_vma+0x4e/0xb0
Code: de 48 89 ef e8 d3 94 fe ff 48 85 c0 74 34 48 83 c4 08 5b 5d c3 48 8d bf 28 01 00 00 be ff ff ff ff e8 d6 46 8b 00 85 c0 75 c8 <0f> 0b 48 8b 85 b8 00 00 00 48 85 c0 75 c6 48 89 ef e8 12 26 87 00
RSP: 0018:ffffc900013df980 EFLAGS: 00010246
RAX: 0000000000000000 RBX: 00007f9df2b80000 RCX: 0000000000000000
RDX: 0000000000000001 RSI: ffffffff822e314c RDI: ffffffff8233c83f
RBP: ffff88811bafc840 R08: ffff888107d0ddb8 R09: 00000000fffffffe
R10: 0000000000000001 R11: 00000000ffbae7ba R12: 0000000000000000
R13: 0000000000000000 R14: ffff88812a710000 R15: ffff888114fa42c0
FS:  00007f9def9d4c00(0000) GS:ffff888266580000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f799627fe50 CR3: 000000011bbc2006 CR4: 00000000003706e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __igt_mmap+0xe0/0x490 [i915]
 igt_mmap+0xd2/0x160 [i915]
 ? __trace_bprintk+0x6e/0x80
 __i915_subtests.cold.7+0x42/0x92 [i915]
 ? i915_perf_selftests+0x20/0x20 [i915]
 ? __i915_nop_setup+0x10/0x10 [i915]
 __run_selftests.part.3+0x10d/0x172 [i915]
 i915_live_selftests.cold.5+0x1f/0x47 [i915]
 i915_pci_probe+0x93/0x1d0 [i915]
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Closes: https://gitlab.freedesktop.org/drm/intel/issues/4129
Link: https://patchwork.freedesktop.org/patch/msgid/20210915105946.394412-1-maarten.lankhorst@linux.intel.comReviewed-by: Matthew Auld <matthew.auld@intel.com>

drivers/gpu/drm/i915/gem/selftests/i915_gem_mman.c

@@ -903,7 +903,9 @@ static int __igt_mmap(struct drm_i915_private *i915,
 
 	pr_debug("igt_mmap(%s, %d) @ %lx\n", obj->mm.region->name, type, addr);
 
+	mmap_read_lock(current->mm);
 	area = vma_lookup(current->mm, addr);
+	mmap_read_unlock(current->mm);
 	if (!area) {
 		pr_err("%s: Did not create a vm_area_struct for the mmap\n",
 		       obj->mm.region->name);