linux: statvfs: allocate spare for f_type

linux: statvfs: allocate spare for f_type

This is the only missing part in struct statvfs.
The LSB calls [f]statfs() deprecated, and its weird types are definitely
off-putting. However, its use is required to get f_type.

Instead, allocate one of the six spares to f_type,
copied directly from struct statfs.
This then becomes a small glibc extension to the standard interface
on Linux and the Hurd, instead of two different interfaces, one of which
is quite odd due to being an ABI type, and there no longer is any reason
to use statfs().

The underlying kernel type is a mess, but all architectures agree on u32
(or more) for the ABI, and all filesystem magicks are 32-bit integers.
We don't lose any generality by using u32, and by doing so we both make
the API consistent with the Hurd, and allow C++
  switch(f_type) { case RAMFS_MAGIC: ...; }

Also fix tst-statvfs so that it actually fails;
as it stood, all it did was return 0 always.
Test statfs()' and statvfs()' f_types are the same.

Link: https://lore.kernel.org/linux-man/f54kudgblgk643u32tb6at4cd3kkzha6hslahv24szs4raroaz@ogivjbfdaqtb/t/#u
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>