rbd: avoid Wreturn-type warnings

In some configurations gcc cannot see that rbd_assert(0) leads to an
unreachable code path:

  drivers/block/rbd.c: In function 'rbd_img_is_write':
  drivers/block/rbd.c:1397:1: error: control reaches end of non-void function [-Werror=return-type]
  drivers/block/rbd.c: In function '__rbd_obj_handle_request':
  drivers/block/rbd.c:2499:1: error: control reaches end of non-void function [-Werror=return-type]
  drivers/block/rbd.c: In function 'rbd_obj_handle_write':
  drivers/block/rbd.c:2471:1: error: control reaches end of non-void function [-Werror=return-type]

As the rbd_assert() here shows has no extra information beyond the verbose
BUG(), we can simply use BUG() directly in its place.  This is reliably
detected as not returning on any architecture, since it doesn't depend
on the unlikely() comparison that confused gcc.

Fixes: 3da691bf ("rbd: new request handling code")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>