Newly mkfs-ed filesystems from Seventh Edition have last modification
time set to zero, but are otherwise perfectly valid.

Also, tighten up other sanity checks to filter out most filesystems with
different bytesex than we're using.

Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: NLubomir Rintel <lkundrak@v3.sk>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>