files: use PROC_SELF when a process accesses its /proc/PID

If a process is executed in another pidns, a /proc/PID doesn't link with
the proper process.

This patch fixes a problem like this:
   1: Error (util.c:106): Unable to close fd 33: Bad file descriptor

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>