ceph: switch netfs read ops to use rreq->inode instead of rreq->mapping->host

One fewer pointer dereference, and in the future we may not be able to
count on the mapping pointer being populated (e.g. in the DIO case).

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>