librbd: Add option to allow disabling issuing alloc hint

librbd: Add option to allow disabling issuing alloc hint

For HDD osd backend, a alloc hint is good for local filesystem to ensure
extent continuous. But for SSD, the available capacity is useful and
precious, user may not expect lots of capacity is used by reserving.
Then ssd is good for random rw.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>