selftests: mptcp: add the MP_FASTCLOSE mibs check

This patch added a new function chk_fclose_nr() to check the numbers
of the MP_FASTCLOSE sending and receiving mibs.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>