Fix UNC parsing on mount

The code cleanup of cifs_parse_mount_options resulted in a new bug being
introduced in the parsing of the UNC. This results in vol->UNC being
modified before vol->UNC was allocated.

Reported-by: Steve French <smfrench@gmail.com>
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>