os/bluestore: provide a different name for fallback allocator

os/bluestore: provide a different name for fallback allocator

Originally primary Hybrid allocator provided its own name when creating a
secondary fallback allocator. This resulted in duplicate admin socket
command registrations for both allocator. Registration return code was
ignored and henoe nobody was aware of the issue. Nautilus might suffer
from the issue though since it asserts on command deregistration failure.
And duplicate name causes such a failure for the secode
unregister_command() call.

Fixes: https://tracker.ceph.com/issues/47443
Signed-off-by: Igor Fedotov <ifedotov@suse.com>