fixed statfs.f_type signed vs unsigned comparisons

statfs.f_type is signed but the filesystem magics are unsigned.
Casting the magics to signed will not make the signed.

Problem seen on big-endian 64bit s390x with __fsword_t 8 bytes.

Casting statfs.f_type to unsigned on the other hand will get us what we
need.

https://bugzilla.redhat.com/show_bug.cgi?id=953217