block: loop: prevent bdev freeing while device in use

TEST:
dd if=/dev/zero of=loop.file bs=1M count=1
while [ true ]; do
   losetup /dev/loop0 loop.file
   echo 2 > /proc/sys/vm/drop_caches
   losetup -d /dev/loop0
done

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>