wireguard: queueing: do not account for pfmemalloc when clearing skb header

Before 8b700862 ("net: Don't copy pfmemalloc flag in __copy_skb_
header()"), the pfmemalloc flag used to be between headers_start and
headers_end, which is a region we clear when preparing the packet for
encryption/decryption. This is a parameter we certainly want to
preserve, which is why 8b700862 moved it out of there. The code here
was written in a world before 8b700862, though, where we had to
manually account for it. This commit brings things up to speed.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>