LU-14391 lnet: optimize the Netlink packet size for routes

LU-14391 lnet: optimize the Netlink packet size for routes

Currently Netlink by default sets its maximum packet size
to send back to user land to 64K. Some sites setup many
routes, above ~430, which exceed this limit. We can avoid
this limitation by calculate about the actually size of
the netlink packet and setting cb->min_dump_alloc. The
new max is then 4GB which should be plenty (27K of routes)

Test-Parameters: trivial testlist=sanity-lnet
Change-Id: Ica01f0cf290992a5d27b8ac2d09508d0a6e8151a
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54844
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>