fstests: replace hexdump with od command

fstests: replace hexdump with od command

The "od" is one of the most fundamental commands in GNU/Linux and
most Unix-like systems. So we nearly always can count on it, don't
need to check if it's installed.

The "hexdump" isn't such fundamental as "od", some systems don't
install it by default. And as "od" nearly can replace all functions
of "hexdump", so let's use an unified command "od" to do the hexdump
job in fstests cases.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>