md/bitmap: fix calculation of 'chunks' - missing shift.

commit 61a0d80c "md/bitmap: discard CHUNK_BLOCK_SHIFT macro"
replaced CHUNK_BLOCK_RATIO() by the same text that was
replacing CHUNK_BLOCK_SHIFT() - which is clearly wrong.

The result is that 'chunks' is often too small by 1,
which can sometimes result in a crash (not sure how).

So use the correct replacement, and get rid of CHUNK_BLOCK_RATIO
which is no longe used.

Reported-by: Karl Newman <siliconfiend@gmail.com>
Tested-by: Karl Newman <siliconfiend@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/bitmap.c

@@ -1727,8 +1727,7 @@ int bitmap_create(struct mddev *mddev)
 	bitmap->chunkshift = (ffz(~mddev->bitmap_info.chunksize)
 			      - BITMAP_BLOCK_SHIFT);
 
-	/* now that chunksize and chunkshift are set, we can use these macros */
-	chunks = (blocks + bitmap->chunkshift - 1) >>
+	chunks = (blocks + (1 << bitmap->chunkshift) - 1) >>
 			bitmap->chunkshift;
 	pages = (chunks + PAGE_COUNTER_RATIO - 1) / PAGE_COUNTER_RATIO;