libceph: use BUG() instead of BUG_ON(1)

I ran into this compile warning, which is the result of BUG_ON(1)
not always leading to the compiler treating the code path as
unreachable:

    include/linux/ceph/osdmap.h: In function 'ceph_can_shift_osds':
    include/linux/ceph/osdmap.h:62:1: error: control reaches end of non-void function [-Werror=return-type]

Using BUG() here avoids the warning.

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