pipe: don't reopen pipes via /proc, if it's not required

The system call open can add O_LARGEFILE.

A point is reopend for unsharing a file descriptor. So here are two
types of points. One is returned by pipe() and all other ones are
got via opening /proc/PID/fd.

Currently I know only one difference between these types, it's
O_LARGEFILE.

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