kern: wg: add support for removing Allowed-IPs

This was recently added to Linux to improve incremental update support,
as you could previously add Allowed-IPs but not remove without replacing
the whole set (and thus, potentially disrupting existing traffic).

Removal is incredibly straightforward; we'll find it in p_aips first
to ensure that it's actually valid for this peer, then we'll delete it
from the radix tree before we remove the corresponding p_aips entry.

Reviewed by: Jason A. Donenfeld, jhb

(cherry picked from commit d15d610fac97df4fefed3f14b31dcfbdcec65bf9)
(cherry picked from commit d1ac3e245f084ee0637bde9a446687621358c418)