io/tst-statvfs: fix statfs().f_type comparison test on some arches

On i686 f_type is an i32 so the test fails when that has the top bit set.

Explicitly cast to u32.

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Florian Weimer <fweimer@redhat.com>