netfilter: nfnetlink: cleanup for nfnetlink_rcv_msg() function

netfilter: nfnetlink: cleanup for nfnetlink_rcv_msg() function

This patch cleans up the message handling path in two aspects:

 * it uses NLMSG_LENGTH() instead of NLMSG_SPACE() like rtnetlink
does in this case to check if there is enough room for the
Netlink/nfnetlink headers. No need to check for the padding room.

 * it removes a redundant header size checking that has been
 already do at the beginning of the function.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>