Unlock inp when handling TCP_MD5SIG socket options

Unlock the inp when hanlding TCP_MD5SIG socket options. tcp_ipsec_pcbctl
handles locking the inp when the option is being modified.

This was found by Claudio Jeker while working on the OpenBGPd port.

On 14 we get a panic when trying to call getsockopt, on 13.1 the process
locks up using 100% CPU.

Reviewed by: rscheff (transport), tuexen
MFC after: 3 days
Sponsored by: Klara Inc.
Differential Revision: https://reviews.freebsd.org/D35532