mutex_lock_interruptible(): add __must_check

It's not sane to use mutex_lock_interruptible() and to then ignore the result.

Ditto down_interruptible(), but I'm lazy.

Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>