net: compat_mmsghdr must be used in sys_recvmmsg

Both to traverse the entries and to set the msg_len field.

Commiter note: folded two patches and avoided one branch repeating the
compat test.

Signed-off-by: Jean-Mickael Guerin <jean-mickael.guerin@6wind.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>