cifs: missed ref-counting smb session in find

When we lookup an smb session based on session id,
we did not up the ref-count for the session. This can
potentially cause issues if the session is freed from under us.

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