netmap: Stop printing a line to the dmesg in netmap_init()
netmap is compiled into the kernel by default so initialization was
always reported, and netmap uses a formatting convention not used in the
rest of the kernel.
Reviewed by: vmaffione
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29099
(cherry picked from commit fef845097190f0ecb783d6c75a9398c4e4a4c0e1)