netmap: Fix driver name handling

[markj]

Summary

if_initname() requires the caller to ensure that the lifetime of the
interface's name buffer contains that of the ifnet itself.
netmap_vi_create() wasn't respecting that; we were instead passing the
stack-allocated buffer provided by the ioctl handler.

While here, add a check to avoid assuming that the caller-provided
buffer is nul-terminated.

Reported by: syzkaller

[vmaffione]

Thanks, but if netmap owns the allocated string (rather than ifnet), shouldn't we deallocate that on deallocation of the netmap_vp_adapter?

[markj]

Free the name buffer in nm_vi_destroy()