r297225 broke udp_output() for the case where the "addr" argument
is NULL and the function jumps to the "release:" label.
For this case, the "inp" was write locked, but the code attempted to
read unlock it. This patch fixes the problem.
This case could occur for NFS over UDP mounts, where the server was
down for a few minutes under certain circumstances.

Reported by: bde
Tested by: bde
Reviewed by: gnn
MFC after: 2 weeks