net: Use kmemdup instead of kmalloc and memcpy

when some other buffer is immediately copied into allocated region.
Replace calls to kmalloc followed by a memcpy with a direct
call to kmemdup.

Signed-off-by: Shyam Saini <mayhs11saini@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>