Allow the buffer_foo() predicates to take a (const struct buffer_head *).

I've checked that the argument of test_bit is indeed "const" on all
architectures.
Signed-off-by: Werner Almesberger <werner@almesberger.net>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>