Bluetooth: Silence DEBUG_STRICT_USER_COPY_CHECKS=y warning

Enabling DEBUG_STRICT_USER_COPY_CHECKS causes the following
warning:

In function 'copy_from_user',
    inlined from 'rfcomm_sock_setsockopt' at
    net/bluetooth/rfcomm/sock.c:705:
arch/x86/include/asm/uaccess_64.h:65:
warning: call to 'copy_from_user_overflow' declared with
attribute warning: copy_from_user() buffer size is not provably
correct

presumably due to buf_size being signed causing GCC to fail to
see that buf_size can't become negative.

Cc: Marcel Holtmann <marcel@holtmann.org>
Cc: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: NStephen Boyd <sboyd@codeaurora.org>
Signed-off-by: NGustavo F. Padovan <padovan@profusion.mobi>

net/bluetooth/rfcomm/sock.c

@@ -679,7 +679,8 @@ static int rfcomm_sock_setsockopt(struct socket *sock, int level, int optname, c
 {
 	struct sock *sk = sock->sk;
 	struct bt_security sec;
-	int len, err = 0;
+	int err = 0;
+	size_t len;
 	u32 opt;
 
 	BT_DBG("sk %p", sk);