fix can not disable max_size quota

Currently if we enable quota and set max_size = -1, it doesn’t
mean max_size is unlimited as expected. Instead, it means object
with any size is not allowed to upload because of “QuotaExceeded”.
The root cause is the function rgw_rounded_kb which convert max_size
to max_size_kb returns 0 for -1 because it takes an unsigned int
but we pass an int to it. A simple fix is check max_size before
it’s rounded to max_size_kb.

Test case:
1 enable and set quota:
radosgw-admin quota enable --uid={user_id} --quota-scope=user
radosgw-admin quota set --quota-scope=user --uid={user_id}\
 --max-objects=100 --max-size=-1
2 upload any object with non-zero length
it will return 403 with “QuotaExceeded” and return 200 if you apply the fix.

Fixes: #9907
Backport: giant, firefly
Signed-off-by: Dong Lei leidong@yahoo-inc.com