xfs: factor out xfs_attr3_leaf_init

Source kernel commit: e65bb55d7f8c2041c8fdb73cd29b0b4cad4ed847

Factor out wrapper xfs_attr3_leaf_init function, which exported for
external use.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Long Li <leo.lilong@huawei.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>