libblkid: use mmap() rather than read()

libblkid: use mmap() rather than read()

diff between
	perf stat -e 'syscalls:sys_enter_*'
for old and new version:

-                35      syscalls:sys_enter_lseek
-                38      syscalls:sys_enter_read
+                 3      syscalls:sys_enter_read
...
-                19      syscalls:sys_enter_mmap
+                17      syscalls:sys_enter_mmap

-       0.001083084 seconds time elapsed
+       0.000751722 seconds time elapsed

The patch dramatically reduces malloc()+seek()+read() operations in
libblkid. The code mmaps ~2MiB of the begin and the end of the device
and it moves buffers management to kernel.

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