rgw: acl grants num limit

rgw: acl grants num limit

According to AWS S3 in this document[1], an ACL can have up to 100
grants.

If the nums of grants is larger than 100, S3 will return like following:
400
<?xml version="1.0" encoding="UTF-8"?>
<Error><Code>MalformedACLError</Code><Message>The XML you provided was not well-formed or did not validate against our published schema</Message><RequestId>10EC67824572C378</RequestId><HostId>AWL3NnQChs/HCfOTu5MtyEc9uzRuxpYMhmvXQry2CovCcuxO2/tMqY1zGoWOur86ipQt3v/WEiA=</HostId></Error>

Now if the nums of request acl grants is larger than the maximum allowed, rgw will return
like following:
400
<?xml version="1.0" encoding="UTF-8"?><Error><Code>MalformedACLError</Code><Message>The request is rejected, because the acl grants number you requested is larger than the maximum 101 grants allowed in an acl.</Message><BucketName>222</BucketName><RequestId>tx000000000000000000017-00596b5fad-101a-default</RequestId><HostId>101a-default-default</HostId></Error>

The maximum number of acl grants can be configured in config file with the configuration item:

rgw_acl_grants_max_num

[1] http://docs.aws.amazon.com/AmazonS3/latest/dev/acl-overview.html

Signed-off-by: Enming Zhang <enming.zhang@umcloud.com>