selftests: pmtu: Introduce check_pmtu_value()

Introduce and use a function that checks PMTU values against
expected values and logs error messages, to remove some clutter.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>