selftests: rtnetlink: Fix do_test_address_proto()

This selftest was introduced recently in the commit cited below. It misses
several check_err() invocations to actually verify that the previous
command succeeded. When these are added, the first one fails, because
besides the addresses added by hand, there can be a link-local address
added by the kernel. Adjust the check to expect at least three addresses
instead of exactly three, and add the missing check_err's.

Furthermore, the explanatory comments assume that the address with no
protocol is $addr2, when in fact it is $addr3. Update the comments.

Fixes: 6a414fd7

 ("selftests: rtnetlink: Add an address proto test")
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/53a579bc883e1bf2fe490d58427cf22c2d1aa21f.1680102695.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>