cifs: print smb3_fs_context::source when mounting

Print full device name (UNC + optional prefix) from @old_ctx->source
when printing info about mount.

Before patch

  mount.cifs //srv/share/dir /mnt -o ...
  dmesg
  ...
  CIFS: Attempting to mount \\srv\share

After patch

  mount.cifs //srv/share/dir /mnt -o ...
  dmesg
  ...
  CIFS: Attempting to mount //srv/share/dir

Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>