rbd: convert all rbd_assert(0) to BUG()

rbd_assert(0) has caused different issues depending on
the compiler version in the past, so it seems better to avoid it
completely.

Replace the remaining instances.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>