libsystemd-network: ipv4ll probe conflict counter (#5361)

libsystemd-network: ipv4ll probe conflict counter (#5361)

A bug exists where the conflict counter is cleared
regardless of whether or not the next probe attempt leads to
a successful address acquisition. This causes 'bursts' of
MAX_CONFLICTS probes followed by a delay of
RATE_LIMIT_INTERVAL instead of a single probe each
RATE_LIMIT_INTERVAL when beyond MAX_CONFLICTS.

The conflict counter should only be cleared after an
address is successfully acquired. This commit achieves that
goal.

From RFC3927:
A host should maintain a counter of the number of address
conflicts it has experienced in the process of trying to
acquire an address, and if the number of conflicts exceeds
MAX_CONFLICTS then the host MUST limit the rate at which it
probes for new addresses to no more than one new address per
RATE_LIMIT_INTERVAL.  This is to prevent catastrophic ARP
storms in pathological failure cases, such as a rogue host
that answers all ARP probes, causing legitimate hosts to go
into an infinite loop attempting to select a usable address.

Signed-off-by: Jason Reeder <jasonreeder@gmail.com>