Extended attribute updates use the deferred work machinery to manage
state across a chain of smaller transactions.  All previous deferred
work users have employed log intent items and log done items to manage
restarting of interrupted operations, which means that ->create_intent
sets dfp_intent to a log intent item and ->create_done uses that item to
create a log intent done item.

However, xattrs have used the INCOMPLETE flag to deal with the lack of
recovery support for an interrupted transaction chain.  Log items are
optional if the xattr update caller didn't set XFS_DA_OP_LOGGED to
require a restartable sequence.

In other words, ->create_intent can return NULL to say that there's no
log intent item.  If that's the case, no log intent done item should be
created.  Clean up xfs_defer_create_done not to do this, so that the
->create_done functions don't have to check for non-null dfp_intent
themselves.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>