cifs: suppress some implicit-fallthrough warnings

To avoid the warning:

     warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Steve French <stfrench@microsoft.com>