rgw: make rollback refcount tag match

rgw: make rollback refcount tag match

When the refcount does a "get", the tag ends with a null character,
but during a rollback, when the tag is "put" it does not end with a
null character, so the same tag is not being manipulated and the
refcounts are therefore off, preventing future deletion of an rgw
object. This adds the null character to the "put" operation.

This change was originally submitted by lltlo <120680451@qq.com> but
wasn't signed-off.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>