xfs: use helpers to extract xattr op from opflags

xfs: use helpers to extract xattr op from opflags

Create helper functions to extract the xattr op from the ondisk xattri
log item and the incore attr intent item.  These will get more use in
the patches that follow.

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