Remove sbinsertoob(), sbinsertoob_locked(). They violate (and have

Remove sbinsertoob(), sbinsertoob_locked().  They violate (and have
basically always violated) invariannts of soreceive(), which assume
that the first mbuf pointer in a receive socket buffer can't change
while the SB_LOCK sleepable lock is held on the socket buffer,
which is precisely what these functions do.  No current protocols
invoke these functions, and removing them will help discourage them
from ever being used.  I should have removed them years ago, but
lost track of it.

MFC after:	1 week
Prodded almost by accident by:	peter