rgw: fix minimum of unordered bucket listing

rgw: fix minimum of unordered bucket listing

A recent PR made sure that a bucket listing could not request too many
entries at once. It also did a minimum computation for number of
entries. For ordered listing the minimum was 0, as required to pass
all unit tests. However the minimum for unordered listing was left at
1. In order to make ordered and unordered listing behave the same --
with the exception of ordering -- the minimum for unordered listing is
modified to 0.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
(manually cherry picked from 38e02a7157d4c534cc9fd8ccfe20fdd93a9b07a8)

Conflicts:
     src/srgw/rgw_rados.cc
- automated cherry-pick from master failed strangely: made the changes manually