blkid: stop scanning on I/O error

blkid: stop scanning on I/O error

Whenever we fail to read from a device it's pointless to
continue with probing; we should be failing immediately.
Otherwise the system will continue logging I/O errors.

This patch updates the probe functions to return the
negative error number on error and BLKID_PROBE_NONE
if not found.

[kzak@redhat.com: - fix s/return errno/return -errno/]

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Karel Zak <kzak@redhat.com>