os/bluestore: Better handling of deferred write trigger

os/bluestore: Better handling of deferred write trigger

Now deferred write in _do_alloc_write does not depend on blob size,
but on size of extent allocated on disk.
It is now possible to set bluestore_prefer_deferred_size way larger than
bluestore_max_blob_size and still get desired behavior.
Example: for deferred=256K, blob=64K : when op write is 128K both blobs will be
written as deferred. When op write is 256K then all will go as regular write.

Signed-off-by: Adam Kupczyk <akupczyk@redhat.com>