SUNRPC: Fix error handling in svc_setup_socket()

Dan points out that sock_alloc_file() releases @sock on error, but
so do all of svc_setup_socket's callers, resulting in a double-
release if sock_alloc_file() returns an error.

Rather than allocating a struct file for all new sockets, allocate
one only for sockets created during a TCP accept. For the moment,
those are the only ones that will ever be used with RPC-with-TLS.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Fixes: ae0d7770

 ("SUNRPC: Ensure server-side sockets have a sock->file")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>