xfs: Add helper function xfs_attr_try_sf_addname

This patch adds a subroutine xfs_attr_try_sf_addname
used by xfs_attr_set.  This subrotine will attempt to
add the attribute name specified in args in shortform,
as well and perform error handling previously done in
xfs_attr_set.

This patch helps to pre-simplify xfs_attr_set for reviewing
purposes and reduce indentation.  New function will be added
in the next patch.

[dgc: moved commit to helper function, too.]

Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>