SMB3: rename macro CIFS_SERVER_IS_CHAN to avoid confusion

Since older dialects such as CIFS do not support multichannel
the macro CIFS_SERVER_IS_CHAN can be confusing (it requires SMB 3
or later) so shorten its name to "SERVER_IS_CHAN"

Suggested-by: Tom Talpey <tom@talpey.com>
Acked-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>