btrfs/137: Make this test compatible with all supported block sizes

btrfs/137: Make this test compatible with all supported block sizes

For large block sizes like 64k it failed simply because this
test was written with 4k block size in mind.
The first few lines of the error logs are as follows:

     d3dc847171f9081bd75d7a2d3b53d322  SCRATCH_MNT/snap2/bar

     File snap1/foo fiemap results in the original filesystem:
    -0: [0..7]: data
    +0: [0..127]: data

     File snap1/bar fiemap results in the original filesystem:
    ...

Fix this by making the test choose offsets and block size as 64k
which is aligned with all the underlying supported fs block sizes.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Reported-by: Disha Goel <disgoel@linux.ibm.com>
Signed-off-by: Nirjhar Roy (IBM) <nirjhar.roy.lists@gmail.com>
Tested-by: Disha Goel <disgoel@linux.ibm.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>