em(4): fix capability bounds needed to access checksum context.

Ensure the offp capability bounds cover entire struct with checksum fields.

This is needed for CHERI systems to avoid bounds violation trap, as
otherwise offp allowed to dereference 4 bytes of csum_flags field only
so bzero failed.

Tested on ARM Morello.

Reviewed by: kbowling
Discussed with: jrtc27
Sponsored by: CHERI Research Centre
Differential Revision: https://reviews.freebsd.org/D53903