[udp] fix possible mbuf and lock leak in udp_input().

In error case we can leave `inp' locked, also we need to free
mbuf chain m' in the same case. Release the lock and use badunlocked'
label to exit with freed mbuf. Also modify UDP error statistic to
match the IPv6 code.

Remove redundant INP_RUNLOCK() from the `if (last == NULL)' block,
there are no ways to reach this point with locked `inp'.

Obtained from: Yandex LLC
MFC after: 3 days
Sponsored by: Yandex LLC