mptcp: introduce MAPPING_BAD_CSUM

This allow moving a couple of conditional out of the fast path,
making the code more easy to follow and will simplify the next
patch.

Fixes: ae66fb2b

 ("mptcp: Do TCP fallback on early DSS checksum failure")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>