e2fsck: pass2 should not process directory blocks that are impossibly large

e2fsck: pass2 should not process directory blocks that are impossibly large

Currently, directories cannot be fallocated, which means that the only
way they get bigger is for the kernel to append blocks one by one.
Therefore, if we encounter a logical block offset that is too big, we
needn't bother adding it to the dblist for pass2 processing, because
it's unlikely to contain a valid directory block.  The code that
handles extent based directories also does not add toobig blocks to
the dblist.

Note that we can easily cause e2fsck to fail with ENOMEM if we start
feeding it really large logical block offsets, as the dblist
implementation will try to realloc() an array big enough to hold it.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>