Add a trivial linux(4) splice(2) implementation, which simply
returns EINVAL.  Fixes grep (grep-3.1-2build1).
PR:		kern/218699
Reported by:	avos
Reviewed by:	emaste
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25636