netfs: Fix undifferentiation of DIO reads from unbuffered reads

On cifs, "DIO reads" (specified by O_DIRECT) need to be differentiated from
"unbuffered reads" (specified by cache=none in the mount parameters).  The
difference is flagged in the protocol and the server may behave
differently: Windows Server will, for example, mandate that DIO reads are
block aligned.

Fix this by adding a NETFS_UNBUFFERED_READ to differentiate this from
NETFS_DIO_READ, parallelling the write differentiation that already exists.
cifs will then do the right thing.

Fixes: 016dc851 ("netfs: Implement unbuffered/DIO read support")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/3444961.1747987072@warthog.procyon.org.uk


Reviewed-by: "Paulo Alcantara (Red Hat)" <pc@manguebit.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
cc: Steve French <sfrench@samba.org>
cc: netfs@lists.linux.dev
cc: v9fs@lists.linux.dev
cc: linux-afs@lists.infradead.org
cc: linux-cifs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: linux-nfs@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>