nodemask.h wants find_next_zero_bit() to take a const address.  Fix that up
and modify find_next_bit() to suit.

(These functions seem too big to be inlined...)

Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>