sk-unix: Use pr_warn instead of pr_perror if socket path can't be stat'ed

The error message is rather confusing people. In worst case (if
it happened that we need this uncollected socket), criu will
print out real error message later.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>