Reject packets larger than IP_MAXPACKET in linux_sendto() for sockets

Reject packets larger than IP_MAXPACKET in linux_sendto() for sockets
with the IP_HDRINCL option set.  Without this change, a Linux process
with access to a raw socket could cause a kernel panic.  Raw sockets
must be created by root, and are generally not consigned to untrusted
applications; hence, the security implications of this bug are
minimal.  I believe this only affects 6-CURRENT on or after 2005-01-30.

Found by:	Coverity Prevent analysis tool
Security:	Local DOS