cifs: avoid parallel session setups on same channel

After allowing channels to reconnect in parallel, it now
becomes important to take care that multiple processes do not
call negotiate/session setup in parallel on the same channel.

This change avoids that by marking a channel as "in_reconnect".
During session setup if the channel in question has this flag
set, we return immediately.

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