Switching find_task_by_vpid call to pid_task, according to
clean ups in pid module.

Reference:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=17f98dcf6010a1cfd25d179fd0ce77d3dc2685c3Signed-off-by: Omar Ramirez Luna <omar.ramirez@ti.com>