[PATCH] md/bitmap: use set_bit etc for bitmap page attributes

In particular, this means that we use 4 bits per page instead of a whole
unsigned long.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>