xfs: Add support to RENAME_EXCHANGE flag

Adds a new function named xfs_cross_rename(), responsible for
handling requests from sys_renameat2() using RENAME_EXCHANGE flag.

Signed-off-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>