cifs: Set default Netbios RFC1001 server name to hostname in UNC

Windows SMB servers (including SMB2+) which are working over RFC1001
require that Netbios server name specified in RFC1001 Session Request
packet is same as the UNC host name. Netbios server name can be already
specified manually via -o servern= option.

With this change the RFC1001 server name is set automatically by extracting
the hostname from the mount source.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>