samples: bpf: Fix tracex7 error raised on the missing argument

samples: bpf: Fix tracex7 error raised on the missing argument

The current behavior of 'tracex7' doesn't consist with other bpf samples
tracex{1..6}. Other samples do not require any argument to run with, but
tracex7 should be run with btrfs device argument. (it should be executed
with test_override_return.sh)

Currently, tracex7 doesn't have any description about how to run this
program and raises an unexpected error. And this result might be
confusing since users might not have a hunch about how to run this
program.

    // Current behavior
    # ./tracex7
    sh: 1: Syntax error: word unexpected (expecting ")")
    // Fixed behavior
    # ./tracex7
    ERROR: Run with the btrfs device argument!

In order to fix this error, this commit adds logic to report a message
and exit when running this program with a missing argument.

Additionally in test_override_return.sh, there is a problem with
multiple directory(tmpmnt) creation. So in this commit adds a line with
removing the directory with every execution.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210727041056.23455-1-claudiajkang@gmail.com