cifs: move rename and delete-on-close logic into helper function

cifs: move rename and delete-on-close logic into helper function

When a file is still open on the server, we attempt to set the
DELETE_ON_CLOSE bit and rename it to a new filename. When the
last opener closes the file, the server should delete it.

This patch moves this mechanism into a helper function and has
the two places in cifs_unlink that do this procedure call it. It
also fixes the open flags to be correct.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>