cifs: open_cached_dir should not rely on primary channel

open_cached_dir today selects ses->server a.k.a primary channel
to send requests. When multichannel is used, the primary
channel maybe down. So it does not make sense to rely only
on that channel.

This fix makes this function pick a channel with the standard
helper function cifs_pick_channel.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>