rgw: mitigate bucket list with max-entries excessively high

rgw: mitigate bucket list with max-entries excessively high

When listing a bucket with radosgw-admin, the user can specify the
maximum number of entries. That number can be unreasonably large, and
can affect the performance and memory availability. For example:

    radosgw-admin bucket list --bucket mybucket1 --max-entries=10000000

This has the potential for creating large data structures at multiple
levels in the the call stack of the radosgw(-admin) process,
potentially causing the process to run out of memory. This change
limits the maximum number of entries requested in all but the high
level code to help mitigate this issue.

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