rgw: don't send tail to gc if copying object to itself

Fixes: #4776
Backport: bobtail
Need to make sure that when copying an object into itself we don't
send the tail to the garbage collection.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
(cherry picked from commit de5d1da810732ee48f41e8be18257053d862301b)