LU-9510 ldiskfs: to not verify preallocation in umount path

LU-9510 ldiskfs: to not verify preallocation in umount path

At umount time while discarding inode preallocation space, panic
occurred due to mismatch found in preallocation space free blocks
i.e pa->pa_free and free blocks calculated by reading on disk
block bitmap within preallocation space length. Similar crash will
occur when user sets errors=panic in mount option and if there is
mismatch in pa space free blocks.

Changes added to not verify mismatch in disk and in-memory
preallocated space unused blocks if the file system is being
umounted.

Seagate-bug-id: MRP-3741
Signed-off-by: Jadhav Vikram <vikramjadhav87@yahoo.co.in>
Signed-off-by: Jadhav Vikram <jadhav.vikram@seagate.com>
Reviewed-by: Alexey Leonidovich Lyashkov <alexey.lyashkov@seagate.com>
Reviewed-by: Ashish Purkar <ashish.purkar@seagate.com>
Tested-by: Elena V. Gryaznova <elena.gryaznova@seagate.com>
Change-Id: I6d43905d49a219d1a5b966ab405e974a1f29b2f3
Reviewed-on: https://review.whamcloud.com/27130
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>