unix: Fix a socket refcount leak in uipc_sendfile_wait()

Fixes: d15792780760 ("unix: new implementation of unix/stream & unix/seqpacket")
Reviewed by: glebius
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D57967

(cherry picked from commit 2a4b8fd1dc070e71fbaab6f12c1c9e4452652aa7)