linsysfs(4): Refactor to avoid referencing an unstable interfaces

Enter the net epoch when traversing a list of interfaces. For that
split the ifname_linux_to_bsd() function on two counterparts, where
the ifname_linux_to_ifp() intended to use in epoch, while the
ifname_linux_to_bsd() intended to be a self-contained.
Until the linux_ioctl_coket() function is refactored, the
ifname_linux_to_bsd() temporarily returns interface outside
of the net epoch.

Reviewed by: melifaro
Differential Revision: https://reviews.freebsd.org/D38790