libceph: support for alloc hint flags

Allow indicating future I/O pattern via flags.  This is supported since
Kraken (and bluestore persists flags together with expected_object_size
and expected_write_size).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>