Don't want to go overboard with the checks, but this is simple and
reasonable.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (modified)
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>