cxgbe: don't support MJUMPAGESIZE != MCLBYTES

[brooks]

Summary

This could only happen on systems with PAGE_SIZE < 4K and FreeBSD
doesn't support such systems.

Sponsored by: DARPA, AFRL

[imp]

Some history: Only the VAX ever had a page size smaller than 4k. It had a 512 byte page. Even 4BSD combined these into 1k or 2k logical page sizes that the system. NetBSD uses 4k logical pages these days.
68k, 88k, pc532, mips, powerpc, sparc, alpha, x86, arm, aarch64, pa-risc, and others designer ones that Linux supports that I surveyed have 4k or larger pages.

So there's very few systems with this page size among the traditional targets of BSD, let alone the rather limited subset that FreeBSD works on today, or is even remotely expected to work on in the future.