syscall: add a sync.Pool for a NetlinkRIB scratch buffer

syscall: add a sync.Pool for a NetlinkRIB scratch buffer

The syscall.NetlinkRIB signature means it necessarily allocates but
this changes makes it allocate a bit less. For example, in the net
package:

    name                           old time/op    new time/op    delta
    Interfaces-8                     59.1µs ± 3%    57.3µs ± 2%   -3.02%  (p=0.000 n=10+10)
    InterfaceByIndex-8               53.1µs ± 5%    50.6µs ± 2%   -4.81%  (p=0.000 n=10+10)
    InterfaceByName-8                59.3µs ± 2%    57.4µs ± 1%   -3.29%  (p=0.000 n=10+10)
    InterfaceAddrs-8                  105µs ± 1%     101µs ± 1%   -4.22%  (p=0.000 n=10+9)
    InterfacesAndAddrs-8             38.2µs ± 3%    36.3µs ± 2%   -4.98%  (p=0.000 n=10+10)
    InterfacesAndMulticastAddrs-8     152µs ± 2%     154µs ± 6%     ~     (p=0.105 n=10+10)

    name                           old alloc/op   new alloc/op   delta
    Interfaces-8                     22.8kB ± 0%    18.7kB ± 0%  -17.90%  (p=0.000 n=10+9)
    InterfaceByIndex-8               16.2kB ± 0%    12.1kB ± 0%  -25.22%  (p=0.000 n=10+10)
    InterfaceByName-8                22.8kB ± 0%    18.8kB ± 0%  -17.85%  (p=0.000 n=10+10)
    InterfaceAddrs-8                 36.6kB ± 0%    28.5kB ± 0%  -22.28%  (p=0.000 n=10+10)
    InterfacesAndAddrs-8             9.60kB ± 0%    5.51kB ± 0%  -42.59%  (p=0.000 n=10+7)
    InterfacesAndMulticastAddrs-8     141kB ± 0%     141kB ± 0%     ~     (all equal)

    name                           old allocs/op  new allocs/op  delta
    Interfaces-8                       53.0 ± 0%      52.0 ± 0%   -1.89%  (p=0.000 n=10+10)
    InterfaceByIndex-8                 28.0 ± 0%      27.0 ± 0%   -3.57%  (p=0.000 n=10+10)
    InterfaceByName-8                  54.0 ± 0%      53.0 ± 0%   -1.85%  (p=0.000 n=10+10)
    InterfaceAddrs-8                    155 ± 0%       153 ± 0%   -1.29%  (p=0.000 n=10+10)
    InterfacesAndAddrs-8               38.0 ± 0%      37.0 ± 0%   -2.63%  (p=0.000 n=10+10)
    InterfacesAndMulticastAddrs-8      77.0 ± 0%      77.0 ± 0%     ~     (all equal)

Signed-off-by: Brad Fitzpatrick <bradfitz@golang.org>
Change-Id: Ic3278b2c000af78d7ed816645463c3b7ff0c90ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/485455
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>