common: handle old versions of modprobe which don't support --help

common: handle old versions of modprobe which don't support --help

There exists versions of modprobe out there which are so old that
modprobe --help isn't a thing.  They're certainly not going to support
modprobe --remove-patiently, so test to make sure modprobe --help
works to avoid causing all tests to fail due to the error message
to stderr showing up in $seq.out.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Fixes: 405c21d40aa1 ("common/module: add patient module rmmod support")
Reported-by: Leah Rumancik <leah.rumancik@gmail.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>