xfs: Refactor xfs_attr_try_sf_addname

To help pre-simplify xfs_attr_set_args, we need to hoist transaction
handling up, while modularizing the adjacent code down into helpers. In
this patch, hoist the commit in xfs_attr_try_sf_addname up into the
calling function, and also pull the attr list creation down.

Signed-off-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Chandan Rajendra <chandanrlinux@gmail.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Acked-by: Dave Chinner <dchinner@redhat.com>