Rename the 'flags' argument to getfsstat() to 'mode' and validate it.

This argument is not a bitmask of flags, but only accepts a single value.
Fail with EINVAL if an invalid value is passed to 'flag'. Rename the
'flags' argument to getmntinfo(3) to 'mode' as well to match.

This is a followup to r308088.

Reviewed by: kib
MFC after: 1 month