generic/775: Fix an infinite loop due to variable name clash

generic/775: Fix an infinite loop due to variable name clash

We use i as the iteration variable in the main test loop as well as some
internal loops. Due to this clash, $i variable of main test loops was
getting modified by the following loop in prep_mixed_mapping().

	for ((i=0; i<num_blocks; i++)); do

If num_blocks is less than 10 (example ext4 with blocksize 4k and
cluster size 8k) i would always be set as 3 and the main loop would
never exit. Use local variable to fix this and while we are at it,
rename it for clarity.

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