family registers all commands RTM_MAX would point to the first entry
of the block following this one and the rtnetlink receive path would accept
a message type for a nonexisting family.

This patch changes RTM_MAX to point to the maximum valid message type
by aligning it to the start of the next block and subtracting one.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
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