linux: require vnet(9) context in ifname_bsd_to_linux_name()

This function is used by netlink(9) only. The netlink(9) taskqueue thread
runs in the vnet of the socket whose request the thread is processing
right now. This is a correct vnet and resetting it to vnet0 is incorrect.
If the function is to be used by any other caller in addition to
netlink(9), it would be caller's responsiblity to provide correct vnet(9).

Reviewed by: melifaro, dchagin
Differential Revision: https://reviews.freebsd.org/D44191
PR: 277286