selftests: pmtu: Move running of test into a new function

Move the block of code that runs a test and prints the verdict to a
new function, run_test.

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