blkis: fix detection of ext4dev as ext4

blkis: fix detection of ext4dev as ext4

If only ext4 is available (as a module or in /proc/filesystems)
blkid wasn't properly testing for it, because the time checks
were backwards and always failed.  This caused old ext4dev
filesystems to fail to mount as ext4.  With this patch it works
fine.

Also, don't try to check for modules on a non-Linux system.

[kzak@redhat.com: port from e2fsprogs to util-linux-ng tree]

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Karel Zak <kzak@redhat.com>