wireguard: socket: remove bogus __be32 annotation

The endpoint->src_if4 has nothing to do with fixed-endian numbers; remove
the bogus annotation.

This was introduced in
https://git.zx2c4.com/wireguard-monolithic-historical/commit?id=14e7d0a499a676ec55176c0de2f9fcbd34074a82


in the historical WireGuard repo because the old code used to
zero-initialize multiple members as follows:

    endpoint->src4.s_addr = endpoint->src_if4 = fl.saddr = 0;

Because fl.saddr is fixed-endian and an assignment returns a value with the
type of its left operand, this meant that sparse detected an assignment
between values of different endianness.

Since then, this assignment was already split up into separate statements;
just the cast survived.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>