xfs: add a xattr_entry helper

Add a helper to translate from the item list head to the attr_intent
item structure and use it so shorten assignments and avoid the need for
extra local variables.

Inspired-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>