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
While here also cleanup some surrounding code; particularly
drop some malloc() casts.
Found with devel/coccinelle.
Reviewed by: bde (previous version - all new bugs are mine)
