tcp_fill_info(): Change lock assertion on INPCB to locked only

This function actually only ever reads from the TCP PCB. Consequently,
also make the pointer to its TCP PCB parameter const.

Sponsored by: NetApp, Inc. (originally)

(cherry picked from commit 8c6104c48ea3f0ffaabeb9784b0f2327db04e7af)

Conflicts (due tue missing 9eb0e832):
sys/dev/cxgbe/tom/t4_tom.c
sys/netinet/tcp_offload.c
sys/netinet/tcp_usrreq.c