tests: mkfs-endianness test use iflag=fullblock to fill block completely with string

tests: mkfs-endianness test use iflag=fullblock to fill block completely with string

When run mkfs-endianness test on lowend machine, sometimes dd didn't
fill block with string then test failed.
So, it's nice to add iflag=fullblock option to fill a block with
string read from stdin.

Tested on Raspberry Pi 3 B+(using 1core to simulate lowend environment)
 with iflag=fullblock option didn't get test failure.

Without iflag=fullblock option.
$ sudo sh -c 'for i in $(seq 1 500); do taskset -c 0 ./ts/cramfs/mkfs-endianness ;  done' | grep FAILED | wc -l
49

With iflag=fullblock option.
$ sudo sh -c 'for i in $(seq 1 500); do taskset -c 0 ./ts/cramfs/mkfs-endianness ;  done' | grep FAILED | wc -l
0

Signed-off-by: Masami Ichikawa <masami256@gmail.com>