netlink: allow to override sb_max for netlink sockets.

Netlink sockets sometimes require larger buffers than other sockets.
For example, full-view IPv4 dump sent via netlink may consume 50+ megabytes.
The desired buffer may be bigger than the system maximum sb_max.
FreeBSD HEAD has a mechanism for overriding protocol setsockopt() handler,
which netlink uses to permit buffer reservations beyond sb_max.
This behaviour is handy as it doesn't require system administrator to lift
the limits for other sockets.
Stable/13 lack such muchanism, so a different approach has to be used.
This diff is a direct commit to stable/13 and it simply copies the logic from
Netlink in -HEAD for checking if the larger buffer size should be permitted.

Differential Revision: https://reviews.freebsd.org/D38472