xfs: Convert xfs_attr_sf macros to inline functions

xfs_attr_sf_totsize() requires access to xfs_inode structure, so, once
xfs_attr_shortform_addname() is its only user, move it to xfs_attr.c
instead of playing with more #includes.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>