ext4/061,062: Minor fixes and refactoring

ext4/061,062: Minor fixes and refactoring

Fix 2 issues in the tests:

1. Use fs atomic write limits instead of bdev's

The tests use block device's limits instead of limits advertised
by filesystem. This can cause failures because block device might
advertise a higher maximum than the FS.  Fix the tests to use
filesystem's limits instead.

2. Fix the test loop range

Fix the test loop start and end so that our calculations for
blocksize, clustersize and iosize are correct.

Reported-by: Disha Goel <disgoel@linux.ibm.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>