cifs: Only pick a channel once per read request

In cifs, only pick a channel when setting up a read request rather than
doing so individually for every subrequest and instead use that channel for
all.  This mirrors what the code in v6.9 does.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Paulo Alcantara <pc@manguebit.com>
cc: Jeff Layton <jlayton@kernel.org>
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>