fusefs: Fix the size of fuse_getattr_in

In FUSE protocol 7.9, the size of the FUSE_GETATTR request has increased.
However, the fusefs driver is currently not sending the additional fields.
In our implementation, the additional fields are always zero, so I there
haven't been any test failures until now. But fusefs-lkl requires the
request's length to be correct.

Fix this bug, and also enhance the test suite to catch similar bugs.

PR: 239830
MFC after: 2 weeks
MFC-With: 350665
Sponsored by: The FreeBSD Foundation