proc: Don't use parent proc_self_fd cached descriptor

When we call open_proc(PROC_SELF, ...) the /proc/self descriptor is
cached in criu. If the process fork()-s after than and child goes
open_proc(PROC_SELF, ...) then it will get the parent's proc descriptor.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>