Reduce overhead of entropy collection

• move harvest mask check inline
• move harvest mask to frequently_read out of actively modified cache line
• disable ether_input collection and describe its limitations in NOTES

Typically entropy collection in ether_input was stirring zero
in to the entropy pool while at the same time greatly reducing
max pps. This indicates that perhaps we should more closely
scrutinize how much entropy we're getting from a given source
as well as what our actual entropy collection needs are for
seeding Yarrow.

Reviewed by: cem, gallatin, delphij
Approved by: secteam
Differential Revision: https://reviews.freebsd.org/D15526