cifs: do not negotiate session if session already exists

In cifs_get_smb_ses, if we find an existing matching session,
we should not send a negotiate request for the session if a
session reconnect is not necessary.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>