CIFS: make mknod() an smb_version_op

This cleanup removes cifs specific code from SMB2/SMB3 code paths
which is cleaner and easier to maintain as the code to handle
special files is improved.  Below is an example creating special files
using 'sfu' mount option over SMB3 to Windows (with this patch)
(Note that to Samba server, support for saving dos attributes
has to be enabled for the SFU mount option to work).

In the future this will also make implementation of creating
special files as reparse points easier (as Windows NFS server does
for example).

   root@smf-Thinkpad-P51:~# stat -c "%F" /mnt2/char
   character special file

   root@smf-Thinkpad-P51:~# stat -c "%F" /mnt2/block
   block special file

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>