xfs: Add attibute remove and helper functions

This patch adds xfs_attr_remove_args. These sub-routines remove
the attributes specified in @args. We will use this later for setting
parent pointers as a deferred attribute operation.

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