e2fsck: force all block allocations to use block_found_map

e2fsck: force all block allocations to use block_found_map

During the later passes of efsck, we sometimes need to allocate and
map blocks into a file.  This can happen either by fsck directly
calling new_block() or indirectly by the library calling new_block
because it needs to allocate a block for lower level metadata (bmap2()
with BMAP_SET; block_iterate3() with BLOCK_CHANGED).

We need to force new_block to allocate blocks from the found block
map, because the FS block map could be inaccurate for various reasons:
the map is wrong, there are missing blocks, the checksum failed, etc.

Therefore, any time fsck does something that could to allocate blocks,
we need to intercept allocation requests so that they're sourced from
the found block map.  Remove the previous code that swapped bitmap
pointers as this is now unneeded.

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