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 <[e-mail]>
Tested-by: Karl Newman <[e-mail]>
Signed-off-by: NeilBrown <[e-mail]>

drivers/md/bitmap.c

···
 	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;
 

drivers/md/bitmap.h

···
 
 #define BITMAP_BLOCK_SHIFT 9
 
-/* how many blocks per chunk? (this is variable) */
-#define CHUNK_BLOCK_RATIO(bitmap) ((bitmap)->mddev->bitmap_info.chunksize >> BITMAP_BLOCK_SHIFT)
-
 #endif
 
 /*