[NET]: Remove unneeded kmalloc() return value casts

Get rid of needless casting of kmalloc() return value in net/

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>