xfs: Factor out xfs_attr_rmtval_invalidate

Because new delayed attribute routines cannot roll transactions, we
carve off the parts of xfs_attr_rmtval_remove that we can use.  This
will help to reduce repetitive code later when we introduce delayed
attributes.

Signed-off-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.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>