mptcp: add more offered MIBs counter

Track the exceptional handling of MPTCP-level offered window
with a few more counters for observability.

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>