seccomp: rework define for SECCOMP_USER_NOTIF_FLAG_CONTINUE

Switch from BIT(0) to (1UL << 0).
First, there are already two different forms used in the header, so there's
no need to add a third. Second, the BIT() macros is kernel internal and
afaict not actually exposed to userspace. Maybe there's some magic there
I'm missing but it definitely causes issues when compiling a program that
tries to use SECCOMP_USER_NOTIF_FLAG_CONTINUE. It currently fails in the
following way:

	# github.com/lxc/lxd/lxd
	/usr/bin/ld: $WORK/b001/_x003.o: in function
	`__do_user_notification_continue':
	lxd/main_checkfeature.go:240: undefined reference to `BIT'
	collect2: error: ld returned 1 exit status

Switching to (1UL << 0) should prevent that and is more in line what is
already done in the rest of the header.

Cc: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20191024212539.4059-1-christian.brauner@ubuntu.comSigned-off-by: Kees Cook <keescook@chromium.org>

include/uapi/linux/seccomp.h

@@ -103,7 +103,7 @@ struct seccomp_notif {
  * SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_TRACE can equally
  * be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.
  */
-#define SECCOMP_USER_NOTIF_FLAG_CONTINUE BIT(0)
+#define SECCOMP_USER_NOTIF_FLAG_CONTINUE (1UL << 0)
 
 struct seccomp_notif_resp {
 	__u64 id;