mbuf: Allow clusters to fill an entire jumbo page.

The assumption that MCLBYTES == MJUMPAGESIZE can only happen if pages
are small is incorrect: it can also happen if MCLSHIFT is adjusted to
increase the size of clusters. Restore the ability to have clusters
fill a jumbo page, while still disallowing them from exceeding them.

MFC after: 1 week
Fixes: 840327e5ddf3, 9c3ad5ba932b
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Reviewed by: siderop1_netapp.com, kevans, brooks
Differential Revision: https://reviews.freebsd.org/D50242

(cherry picked from commit 271128b0698653294acf0ed3457d5871af5b3ef1)