libblkid: revert mmap usage

libblkid: revert mmap usage

The implementation has not been ready for I/O errors and it seems that
there is no elegant way how to resolve this issue. Linux returns
SIGBUS on mmap errors and play with signals (or longjumps) in shared
library is really bad idea.

It also seems that mmaped devices have some unexpected side-effects
with page-cache where for example dd returns old data for already
modified device etc.

Signed-off-by: Karel Zak <kzak@redhat.com>