cifs: refactor mid finding loop in cifs_demultiplex_thread

...to reduce the extreme indentation. This should introduce no
behavioral changes.

Cc: stable@kernel.org
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>