rgw: clear buckets before calling list_buckets()

rgw: clear buckets before calling list_buckets()

The ragodgw-admin bucket limit check command has a bug in
octopus. Since we do not clear the bucket list before
list_buckets() returns the next max_entries, they are appended
to the existing list and we end up counting the first ones again.

This bug is triggered if bucket count exceeds max_entries and
causes duplicates in the output of radosgw-admin bucket limit check.

The fix clears the buckets structure before the list_buckets()
populates it again with the next lot of buckets to iterate through.

partial manual cherry-pick of 99f7c4aa1286edfea6961b92bb44bb8fe22bd599

Signed-off-by: Nikhil Kshirsagar <nkshirsagar@gmail.com>