cifs: Fix server re-repick on subrequest retry

When a subrequest is marked for needing retry, netfs will call
cifs_prepare_write() which will make cifs repick the server for the op
before renegotiating credits; it then calls cifs_issue_write() which
invokes smb2_async_writev() - which re-repicks the server.

If a different server is then selected, this causes the increment of
server->in_flight to happen against one record and the decrement to happen
against another, leading to misaccounting.

Fix this by just removing the repick code in smb2_async_writev().  As this
is only called from netfslib-driven code, cifs_prepare_write() should
always have been called first, and so server should never be NULL and the
preparatory step is repeated in the event that we do a retry.

The problem manifests as a warning looking something like:

 WARNING: CPU: 4 PID: 72896 at fs/smb/client/smb2ops.c:97 smb2_add_credits+0x3f0/0x9e0 [cifs]
 ...
 RIP: 0010:smb2_add_credits+0x3f0/0x9e0 [cifs]
 ...
  smb2_writev_callback+0x334/0x560 [cifs]
  cifs_demultiplex_thread+0x77a/0x11b0 [cifs]
  kthread+0x187/0x1d0
  ret_from_fork+0x34/0x60
  ret_from_fork_asm+0x1a/0x30

Which may be triggered by a number of different xfstests running against an
Azure server in multichannel mode.  generic/249 seems the most repeatable,
but generic/215, generic/249 and generic/308 may also show it.

Fixes: 3ee1a1fc ("cifs: Cut over to using netfslib")
Cc: stable@vger.kernel.org
Reported-by: Steve French <smfrench@gmail.com>
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Acked-by: Tom Talpey <tom@talpey.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Aurelien Aptel <aaptel@suse.com>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/smb2pdu.c

@@ -4859,9 +4859,6 @@ smb2_async_writev(struct cifs_io_subrequest *wdata)
 	struct cifs_io_parms *io_parms = NULL;
 	int credit_request;
 
-	if (!wdata->server || test_bit(NETFS_SREQ_RETRYING, &wdata->subreq.flags))
-		server = wdata->server = cifs_pick_channel(tcon->ses);
-
 	/*
 	 * in future we may get cifs_io_parms passed in from the caller,
 	 * but for now we construct it here...