ext2fs: cleanup generation number management.

Ext2/3/4 manages generation numbers differently than UFS so adopt
some rules that should work well. When allocating a new inode,
make sure we generate a "good" random value specifically avoiding
zero.

Don't interfere with the numbers that are already generated in
the filesystem: ext2fs doesn't have the backwards compatibility
issues where there were no generation numbers.

Reviewed by: kevlo
MFC after: 1 week