e2fsck: in rehash, mark newly allocated extent blocks as found

e2fsck: in rehash, mark newly allocated extent blocks as found

When we're rehashing directories, it's possible that an extent block
(or a map block) could be (silently) allocated by the underlying
libext2fs when expanding the directory.  This silent allocation is not
captured in block_found_map, which is disastrous if later the rehash
process expands another directory and uses that same block from
before without realizing that it's now in use.

Therefore, if we notice that the free block count has dropped by more
than what e2fsck allocated itself during the expansion, we iterate the
directory's blocks a second time to ensure that these silent
allocations are marked in the found blocks bitmap.

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