rgw: fix bad versioned bucket stats after reshard

rgw: fix bad versioned bucket stats after reshard

When a versioned bucket is resharded, the stats for bucket index
entries of type PlainIdx and InstanceIdx were both accumulated to
determine the bucket stats. This caused a doubling of some stats, such
as bytes used. This fix makes certain that only PlainIdx entries are
accumulated. This works for unversioned buckets as well, as they only
have Plain Idx entries.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
(cherry picked from commit 6379ec6a81bf469341a095b3b852678115fb99f7)