nvme: Add constants for the extended data for Get Log Page command flag

nvme(4) doesn't check this flag, but Fabrics implementations may need
to set this flag in the log page attributes cdata field.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44444

(cherry picked from commit cbda1886ab1cd3ec2847b7da5136d3bb68d56101)