Fix ABI in set_mempolicy() that got broken by an earlier change.

Add a check for very big input values and prevent excessive looping in the
kernel.

Cc: Paul "nyer, nyer, your mother wears combat boots!" Jackson <pj@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>