lsns: tolerate lsns_ioctl(fd, NS_GET_{PARENT,USERNS}) failing with ENOSYS

lsns: tolerate lsns_ioctl(fd, NS_GET_{PARENT,USERNS}) failing with ENOSYS

With the original code, "lsns/filedesc" test case failed on
"build (qemu-user, s390x)" and "build (qemu-user, riscv64)".

On the platforms, lsns_ioctl(fd, NS_GET_{PARENT,USERNS}) failed
with ENOSYS. The error stoped the iteration for gathering
information from /proc/[0-9]+. As a result, lsns printed
nothing. We don't expect this behavior.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>