fix: use fragmented block size for space calculation

fix: use fragmented block size for space calculation

We currently calculate the size of a datastore using `statfs64`, which
returns the number of blocks in the fs and the two block sizes:
fragemented block size(f_frsize) and block size (f_bsize). To calculate
eg the total space in a datastore we use total_blocks * f_bsize, which
is not always correct.

`f_frsize` is the minimum unit of allocation on the filesystem (in
bytes) and in 99% of the cases equal to `f_bsize`, but in some cases
it differs. For example some filesystems allow smaller blocks for small
files, in case f_frsize < f_bsize. In that case, f_frsize * total_blocks
returns (mostly) the correct result (ceph also did some weird stuff, which is
now being fixed though [0][1]). `statvfs` also documents this as the
recommended way ('fsblkcnt_t f_blocks;   /* Size of fs in f_frsize units */')[2].

This patch aligns the the behavior with the libc utilities (also used by
`df`) [3].

Motivation: [4] (Forum post)

[0]: https://tracker.ceph.com/issues/3793
[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=92a49fb0f79f3300e6e50ddf56238e70678e4202
[2]: https://www.man7.org/linux/man-pages/man3/statvfs.3.html
[3]: https://git.savannah.gnu.org/cgit/gnulib.git/tree/lib/fsusage.c#n147
[4]: https://forum.proxmox.com/threads/pbs-3-1-2-wrong-datastore-information-sshfs.139875/#post-626959

Signed-off-by: Gabriel Goller <g.goller@proxmox.com>