[PATCH] alpha: Fix getxpid on alpha so it works for threads

While looking in the code I discovered that alpha has fallen behind because
it doesn't use sys_getppid.  The problem is that it doesn't follow the task
struct to the task_group_leader.

Acked-by: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>