rbd: set io_min, io_opt and discard_granularity to alloc_size

Now that we have alloc_size that controls our discard behavior, it
doesn't make sense to have these set to object (set) size.  alloc_size
defaults to 64k, but because discard_granularity is likely 4M, only
ranges that are equal to or bigger than 4M can be considered during
fstrim.  A smaller io_min is also more likely to be met, resulting in
fewer deferred writes on bluestore OSDs.

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