rpc: Improve socket locking in svc_vc_accept()

so_state modifications must be synchronized by the socket lock. For the
listening socket this probably doesn't matter but for the child socket I
think it's possible that this unlocked update clobbers a state
transition if the nascent connection is being disconnected for some
reason.

Also fix the line which potentially clears SS_NBIO in the listening
socket.

It is unclear whether this code is used at all.

Reviewed by: glebius
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D55247