libext2fs: fix free block accounting for 64-bit file systems

libext2fs: fix free block accounting for 64-bit file systems

We rely on a nasty hack to adjust the free block count where we pass
signed value into ext2fs_free_blocks_count_add(), which takes an
64-bit unsigned value, and relies on overflow and C's signed->unsigned
semantics to do the subtraction.  This works, so long as a 64-bit
signed value is used.

Unfortunately, ext2fs_block_alloc_stats2() and
ext2fs_block_alloc_stats_range(), this is not true, so on a 64-bit
file system, the free blocks accounting can get screwed up.

A simple way to demonstrate the problem is:

mke2fs -F -t ext4 -O 64bit /tmp/foo.img 1M
e2fsck -fy /tmp/foo.img

... which will result in the following e2fsck complaint:

Pass 5: Checking group summary information
Free blocks count wrong (4294968278, counted=982).
Fix? yes

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