dm thin: Use last transaction's pmd->root when commit failed

In the Linux kernel, the following vulnerability has been resolved:

dm thin: Use last transaction's pmd->root when commit failed

Recently we found a softlock up problem in dm thin pool btree lookup
code due to corrupted metadata:

Kernel panic - not syncing: softlockup: hung tasks
CPU: 7 PID: 2669225 Comm: kworker/u16:3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
Workqueue: dm-thin do_worker [dm_thin_pool]
Call Trace:
<IRQ>
dump_stack+0x9c/0xd3
panic+0x35d/0x6b9
watchdog_timer_fn.cold+0x16/0x25
__run_hrtimer+0xa2/0x2d0
</IRQ>
RIP: 0010:__relink_lru+0x102/0x220 [dm_bufio]
__bufio_new+0x11f/0x4f0 [dm_bufio]
new_read+0xa3/0x1e0 [dm_bufio]
dm_bm_read_lock+0x33/0xd0 [dm_persistent_data]
ro_step+0x63/0x100 [dm_persistent_data]
btree_lookup_raw.constprop.0+0x44/0x220 [dm_persistent_data]
dm_btree_lookup+0x16f/0x210 [dm_persistent_data]
dm_thin_find_block+0x12c/0x210 [dm_thin_pool]
__process_bio_read_only+0xc5/0x400 [dm_thin_pool]
process_thin_deferred_bios+0x1a4/0x4a0 [dm_thin_pool]
process_one_work+0x3c5/0x730

Following process may generate a broken btree mixed with fresh and
stale btree nodes, which could get dm thin trapped in an infinite loop
while looking up data block:
Transaction 1: pmd->root = A, A->B->C // One path in btree
pmd->root = X, X->Y->Z // Copy-up
Transaction 2: X,Z is updated on disk, Y write failed.
// Commit failed, dm thin becomes read-only.
process_bio_read_only
dm_thin_find_block
__find_block
dm_btree_lookup(pmd->root)
The pmd->root points to a broken btree, Y may contain stale node
pointing to any block, for example X, which gets dm thin trapped into
a dead loop while looking up Z.

Fix this by setting pmd->root in __open_metadata(), so that dm thin
will use the last transaction's pmd->root if commit failed.

Fetch a reproducer in [Link].

Linke: https://bugzilla.kernel.org/show_bug.cgi?id=216790