selftests: mptcp: pm_nl_ctl: fix 32-bit support

When using pm_nl_ctl to validate userspace path-manager's behaviours, it
was failing on 32-bit architectures ~half of the time.

pm_nl_ctl was not reporting any error but the command was not doing what
it was expected to do. As a result, the expected linked event was not
triggered after and the test failed.

This is due to the fact the token given in argument to the application
was parsed as an integer with atoi(): in a 32-bit arch, if the number
was bigger than INT_MAX, 2147483647 was used instead.

This can simply be fixed by using strtoul() instead of atoi().

The errors have been seen "by chance" when manually looking at the
results from LKFT.

Fixes: 9a0b3650 ("selftests: mptcp: support MPTCP_PM_CMD_ANNOUNCE")
Cc: stable@vger.kernel.org
Fixes: ecd2a77d ("selftests: mptcp: support MPTCP_PM_CMD_REMOVE")
Fixes: cf8d0a6d ("selftests: mptcp: support MPTCP_PM_CMD_SUBFLOW_CREATE")
Fixes: 57cc361b ("selftests: mptcp: support MPTCP_PM_CMD_SUBFLOW_DESTROY")
Fixes: ca188a25 ("selftests: mptcp: userspace PM support for MP_PRIO signals")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>