rbd: Use min_t() to fix comparison of distinct pointer types warning

drivers/block/rbd.c: In function ‘zero_pages’:
drivers/block/rbd.c:1102: warning: comparison of distinct pointer types lacks a cast

Remove the hackish casts and use min_t() to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Alex Elder <elder@inktank.com>