Fix a leak in socket(2) when we fail to allocate a file descriptor.

Got broken by "make sock_alloc_file() do sock_release() on failures" -
cleanup after sock_map_fd() failure got pulled all the way into
sock_alloc_file(), but it used to serve the case when sock_map_fd()
failed *before* getting to sock_alloc_file() as well, and that got
lost.  Trivial to fix, fortunately.

Fixes: 8e1611e2

 (make sock_alloc_file() do sock_release() on failures)
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>