rgw:admin/Fix update op mask bug in modify user method

[hasantir]

Checklist

• Added tests for features and functional changes
• Public functions and types are documented
• Standard formatting is applied to Go code
• Is this a new API? Added a new file that begins with //go:build ceph_preview
• Ran make api-update to record new APIs

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[phlogistonjohn]

Please include a few more details so we can validate this change, such as:

1. details about the bug you saw
2. an explanation of why/how this fixes said bug
3. a unit test validating the behavior

We also prefer contributions include a Signed-off-by tag in the git commit. Thanks!