[MIPS] Use find_task_by_vpid in system calls

There are some places left in mips, that lookup task in initial namespace,
while the code doing so gets the pid from the user space and thus must
treat it as virtual.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>