ext4/002: Work with 64k block size

ext4/002: Work with 64k block size

This commit changes the script to operate on FS blocks by obtaining the
block size from the underlying filesystem and using it to perform I/O in
units of block sizes.

This commit also uses $SCRATCH_MNT rather than $TEST_DIR for holding the
test files since the FS on $TEST_DIR might be created with a different
block size than the one specified in $MKFS_OPTIONS.

Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>