IP multicast sysctl mrtmfc must not write outside of allocation.

IP multicast sysctl mrtmfc must not write outside of allocation.

Reading sysctl mrt_sysctl_mfc() allocates memory to be copied back
to user.  Chunks of struct mfcinfo are copied from routing table
to linear heap memory.  If the allocated memory was not a multiple
the struct size, a struct mfcinfo could be copied to a partially
unallocated destination.  Check that the end of the struct is within
the allocation.

From Alfredo Ortega;  OK claudio@