ext4: protect pa->pa_free in ext4_discard_allocated_blocks

If ext4_mb_mark_diskspace_used fails in ext4_mb_new_blocks, we may
discard pa already in list. Protect pa with pa_lock to avoid race.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://lore.kernel.org/r/20230303172120.3800725-7-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>