Fix handling of groups during chown
Fix handling of groups during chown

Ensure the attributes returned from setattr carry the correct GID so
that the group change is immediately visible to the user without having
to wait for the attribute's TTL to expire.

In other words, fix a typo, which in turn fixes the TestReadWrite_Chown
and TestReadWrite_FchownOnDeletedNode integration tests.

bazelbuild/sandboxfs,bazelbuild/sandboxfs,bazelbuild/sandboxfs