xfs: add the ability to join a held buffer to a defer_ops

xfs: add the ability to join a held buffer to a defer_ops

Source kernel commit: b7b2846fe26f2c0d7f317c874a13d3ecf22670ff

In certain cases, defer_ops callers will lock a buffer and want to hold
the lock across transaction rolls.  Similar to ijoined inodes, we want
to dirty & join the buffer with each transaction roll in defer_finish so
that afterwards the caller still owns the buffer lock and we haven't
inadvertently pinned the log.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>