kexec: fix compilation warning on xchg(&kexec_lock, 0) in kernel_kexec()

kernel/kexec.c: In function 'kernel_kexec':
kernel/kexec.c:1506: warning: value computed is not used

Signed-off-by: Huang Ying <ying.huang@intel.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/kexec.c

@@ -1503,7 +1503,8 @@ int kernel_kexec(void)
 	}
 
  Unlock:
-	xchg(&kexec_lock, 0);
+	if (!xchg(&kexec_lock, 0))
+		BUG();
 
 	return error;
 }