rtnetlink: avoid frame size warning in rtnl_newlink()

Standard kernel compilation produces the following warning:

net/core/rtnetlink.c: In function ‘rtnl_newlink’:
net/core/rtnetlink.c:3232:1: warning: the frame size of 1288 bytes is larger than 1024 bytes [-Wframe-larger-than=]
 }
  ^

This should not really be an issue, as rtnl_newlink() stack is
generally quite shallow.

Fix the warning by allocating attributes with kmalloc() in a wrapper
and passing it down to rtnl_newlink(), avoiding complexities on error
paths.

Alternatively we could kmalloc() some structure within rtnl_newlink(),
slave attributes look like a good candidate.  In practice it adds to
already rather high complexity and length of the function.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>