Previously, sorele() always required the socket lock and dropped the
lock if the released reference was not the last reference.  Many
callers locked the socket lock just before calling sorele() resulting
in a wasted lock/unlock when not dropping the last reference.

Move the previous implementation of sorele() into a new
sorele_locked() function and use it instead of sorele() for various
places in uipc_socket.c that called sorele() while already holding the
socket lock.

The sorele() macro now uses refcount_release_if_not_last() try to drop
the socket reference without locking the socket.  If that shortcut
fails, it locks the socket and calls sorele_locked().

Reviewed by:	kib, markj
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D32741