Add a more sophisticated algorithm to e2fsck to salvage corrupted

Add a more sophisticated algorithm to e2fsck to salvage corrupted
directories.

Speed up e2fsck slightly by only updating the master superblock;
there is no point to update the backup superblocks.

Fix a small bug in the rehashing code which could leave the indexed
flag set even after the directory was compressed instead of indexed.
(Not fatal, since the kernel will deal with this, but technically
it filesystem isn't consistent, and the filesystem will be marked
as being in error when the kernel comes across the directory.  It
should also never happen in real life, since directories that small
will never be indexed, but better safe than sorry.)

Also change the threshold of when directories are indexed, so that
directories of size 2 blocks will be indexed.  Otherwise they will
never be indexed by the kernel when they grow.