e2fsck: Handle a pass 2 "should never happen" error gracefully

e2fsck: Handle a pass 2 "should never happen" error gracefully

Turns out a "should never happen" error can indeed happen very easily
if a directory with an htree index has an incorrect, and too-large,
i_size field.  This patch fixes this so that we handle this situation
gracefully, allowing filesystems with this error to be fixed.

In another patch I will clean up the specific problem which caused the
internal "should never happen" error from happening at all, but patch
will prevent e2fsck from crashing, and prompt the user to remove the
htree index, so it can be rebuilt again after pass 3.

Thanks to Bas van Schaik at Tetra for giving me access to his system
so this problem could be debugged.

Addresses-Launchpad-Bug: #129395

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>