support bitmaps, but will make a difference for raid10]

This has the added advantage of meaning with can move the thread->timeout
manipulation inside the bitmap.c code instead of sprinkling identical code
throughout all personalities.

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