m_mpls: test the 'mac_push' action after 'modify'

m_mpls: test the 'mac_push' action after 'modify'

Commit 02a261b5ba1c ("m_mpls: add mac_push action") added a matches()
test for the "mac_push" string before the test for "modify".
This changes the previous behaviour as 'action m' used to match
"modify" while it now matches "mac_push".

Revert to the original behaviour by moving the "mac_push" test after
"modify".

Fixes: 02a261b5ba1c ("m_mpls: add mac_push action")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David Ahern <dsahern@gmail.com>