e1000: Don't use 9k jumbo clusters

Backported to 11-STABLE from 12-CURRENT.
Avoids the issue with 9k jumbo cluster fragmentation
by maxing out at page size jumbo clusters for RX mbufs.

Submitted by: Ryan Moeller
Reviewed by: erj@
Differential Revision: https://reviews.freebsd.org/D16534