mptcp: fix error mibs accounting

The current accounting for MP_FAIL and FASTCLOSE is not very
accurate: both can be increased even when the related option is
not really sent. Move the accounting into the correct place.

Fixes: eb7f3365 ("mptcp: add the mibs for MP_FAIL")
Fixes: 1e75629c ("mptcp: add the mibs for MP_FASTCLOSE")
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>