xfs: formalize the process of holding onto resources across a defer roll

xfs: formalize the process of holding onto resources across a defer roll

Source kernel commit: c5db9f937b2971c78d6c6bbaa61a6450efa8b845

Transaction users are allowed to flag up to two buffers and two inodes
for ownership preservation across a deferred transaction roll.  Hoist
the variables and code responsible for this out of xfs_defer_trans_roll
so that we can use it for the defer capture mechanism.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>