e2fsck: fix bad htree checksums in preen mode

e2fsck: fix bad htree checksums in preen mode

We attempt to fix directories which have a bad/corrupted htree index
node by completely rebuilding the directory htree nodes.  Since this
is a very safe thing to do and has no risk of losing directory
entries, we've enabled this for preen mode.  Unfortunately, subsequent
index nodes look like empty directory entries that fill the entire
block --- without a checksum at the end of the directory.  So these
nodes will be treated as a completely corrupted directory block, and
this will *not* be fixed while in preen mode.

So add code to treat an empty directory entry which covers the entire
block as valid if the directory is already on the list of inodes to be
rebuilt.

Addresses-Gooogle-Bug: 178607853
Signed-off-by: Theodore Ts'o <tytso@mit.edu>