conn: reduce initial memory in Receive()

[florianl]

Reduce memory footprint by starting with a smaller buffer and adjust the buffer size accordingly after peeking at the upcoming netlink message.

// on current master
$ go test -memprofile mem.orig.out .
// on this PR
$ go test  -memprofile mem.new.out .
$ go tool pprof -base=mem.orig.out mem.new.out
(pprof) top
Showing nodes accounting for -1081.85MB, 41.71% of 2593.47MB total
Dropped 34 nodes (cum <= 12.97MB)
Showing top 10 nodes out of 25
      flat  flat%   sum%        cum   cum%
-1226.36MB 47.29% 47.29% -1074.85MB 41.44%  github.com/mdlayher/netlink.(*conn).Receive
   92.51MB  3.57% 43.72%   144.01MB  5.55%  github.com/mdlayher/socket.rwT[go.shape.struct { github.com/mdlayher/socket.n int; github.com/mdlayher/socket.oobn int; github.com/mdlayher/socket.recvflags int; github.com/mdlayher/socket.from golang.org/x/sys/unix.Sockaddr }]
   46.50MB  1.79% 41.93%    51.51MB  1.99%  golang.org/x/sys/unix.Recvmsg
     -19MB  0.73% 42.66%      -19MB  0.73%  github.com/mdlayher/socket.rwT[go.shape.int]
   10.50MB   0.4% 42.25%   154.51MB  5.96%  github.com/mdlayher/socket.(*Conn).Recvmsg
       8MB  0.31% 41.95%      -11MB  0.42%  github.com/mdlayher/socket.(*Conn).Sendmsg
    3.50MB  0.13% 41.81% -1071.35MB 41.31%  github.com/mdlayher/netlink.(*Conn).receive
    2.50MB 0.096% 41.71%    -7.50MB  0.29%  github.com/mdlayher/netlink.(*conn).Send
         0     0% 41.71%  -516.45MB 19.91%  github.com/mdlayher/netlink.(*Conn).Execute
         0     0% 41.71%  -553.40MB 21.34%  github.com/mdlayher/netlink.(*Conn).Receive