cifs: add function to set file disposition

The proper way to set the delete on close bit on an already existing
file is to use SET_FILE_INFO with an infolevel of
SMB_FILE_DISPOSITION_INFO. Add a function to do that and have the
silly-rename code use it.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>