ext4: fix COLLAPSE RANGE test for bigalloc file systems

Blocks in collapse range should be collapsed per cluster unit when
bigalloc is enable. If bigalloc is not enable, EXT4_CLUSTER_SIZE will
be same with EXT4_BLOCK_SIZE.

With this bug fixed, patch enables COLLAPSE_RANGE for bigalloc, which
fixes a large number of xfstest failures which use fsx.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Ashish Sangwan <a.sangwan@samsung.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>