wtap(4): Rename interface name

The original interface created by wtap is named "wlan%d", which is the
same as the name of the vap created by wlan(4) and cause ifconfig(8)
may output like this:

wlan0:
        parent interface: wlan0

Rename the interface created by wtap(4) to "wtap%d" to avoid confusing.

Reviewed by: adrian
Sponsored by: Google, Inc. (GSoC 2022)
Differential Revision: https://reviews.freebsd.org/D35751