xfs: merge xfs_attr_leaf_try_add into xfs_attr_leaf_addname

xfs_attr_leaf_try_add is only called by xfs_attr_leaf_addname, and
merging the two will simplify a following error handling fix.

To facilitate this move the remote block state save/restore helpers up in
the file so that they don't need forward declarations now.

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