mptcp: introduce MAPPING_BAD_CSUM

commit 31bf11de upstream.

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>
[ Conflicts in subflow.c, because commit 0348c690 ("mptcp: add the
  fallback check") is not in this version. This commit is linked to a
  new feature, changing the context around. The new condition can still
  be added at the same place. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>