e2fsck: fix inode coherency issue when iterating an inode's blocks

e2fsck: fix inode coherency issue when iterating an inode's blocks

When we're about to iterate the blocks of a block-map file, we need to
write the inode out to disk if it's dirty because block_iterate3()
will re-read the inode from disk.  (In practice this won't happen
because nothing dirties block-mapped inodes before the iterate call,
but we can program defensively).

More importantly, we need to re-read the inode after the iterate()
operation because it's possible that mappings were changed (or erased)
during the iteration.  If we then dirty or clear the inode, we'll
mistakenly write the old inode values back out to disk!

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