osdc: pass fadvise op flags to WritebackHandler read requests

osdc: pass fadvise op flags to WritebackHandler read requests

librbd was previously attempting to cast the provided Context to
retrieve the fadvise flags.  To eliminate the unsafe cast, now
the fadvise flags are directly passed to the WritebackHandler::read
callback.

Fixes: #10914
Signed-off-by: Jason Dillaman <dillaman@redhat.com>