mbuf: Don't support PAGE_SIZE < 4K
The Vax supported such things, but FreeBSD does not. This further
implies that MJUMPAGESIZE > MCLBYTES so assert this and remove code
handling them being equal.
Reviewed by: kp, imp, jhb
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D36320