selftests: net: Add protodown tests

Add a selftest for the protodown mechanism.

Five test cases are included:

1. Basic protodown toggling: Verify that setting protodown on macvlan
   results in DOWN operational state and clearing it restores UP.

2. Same as the previous test case, but with vxlan.

3. Protodown reasons: Verify that protodown cannot be cleared while
   there are active protodown reasons, but can be cleared once all
   reasons are removed.

4. Protodown with lower device being toggled: Verify that toggling the
   lower device's carrier while protodown is on does not cause the
   macvlan to gain carrier.

5. Protodown with lower device down: Verify that toggling protodown
   while the lower device has no carrier does not cause the macvlan to
   gain carrier.

Note that the last two test cases fail without "net: Do not turn on
carrier when protodown is on" and "net: Do not unconditionally turn on
carrier when turning off protodown":

 # ./protodown.sh
 TEST: Basic protodown on/off with macvlan                           [ OK ]
 TEST: Basic protodown on/off with vxlan                             [ OK ]
 TEST: Protodown reasons                                             [ OK ]
 TEST: Protodown with lower device toggled                           [FAIL]
         Macvlan operational state is not DOWN despite protodown
 TEST: Protodown with lower device down                              [FAIL]
         Macvlan is not LOWERLAYERDOWN after clearing protodown

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260507105906.891817-5-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>