libext2fs: fix 64bit overflow in ext2fs_block_alloc_stats_range

libext2fs: fix 64bit overflow in ext2fs_block_alloc_stats_range

In ext2fs_block_alloc_stats_range(), the quantity "-inuse * n" is
calculated as a signed 32-bit quantity.  Unfortunately, gcc (4.6.3 on
Ubuntu 12.04) doesn't sign-extend this quantity to fill the blk64_t
parameter that ext2fs_free_blocks_count_add() wants, so the end result
is that the superblock gets a ridiculously huge free block count.

Changing the declaration of 'n' to blk64_t seems to fix this.

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