libext2fs: fix arguments passed to ->block_alloc_stats_range

libext2fs: fix arguments passed to ->block_alloc_stats_range

In ext2fs_block_alloc_stats_range, we use @num as the loop counter but
then pass it to the callback and @blk as the loop cursor.  This means
that the range passed to e2fsck_block_alloc_stats_range starts beyond
the range that was actually freed and has a length of zero, which is not
at all correct.

Fix this by saving the original values and passing those instead.

Cc: linux-ext4@vger.kernel.org # v1.43
Fixes: 647e8786156061 ("libext2fs: add new hooks to support large allocations")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/175182663005.1984706.2711154041137486922.stgit@frogsfrogsfrogs
Signed-off-by: Theodore Ts'o <tytso@mit.edu>