crush: always return a value from crush_bucket_choose

Even when we encounter a corrupt bucket.  We still BUG().  This fixes
the warning

fs/ceph/crush/mapper.c: In function 'crush_choose':
fs/ceph/crush/mapper.c:352: warning: control may reach end of non-void function
'crush_bucket_choose' being inlined

Signed-off-by: Sage Weil <sage@newdream.net>