x86/process: Avoid unnecessary NULL check in get_wchan()

Task 'p' is always guaranteed to be non-NULL, because the only call
sites are in fs/proc/ which all guarantee a non-NULL task pointer.

[ mingo: Improved the changelog. ]
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1542798734-12532-1-git-send-email-laoar.shao@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/process.c

@@ -730,7 +730,7 @@ unsigned long get_wchan(struct task_struct *p)
 	unsigned long start, bottom, top, sp, fp, ip, ret = 0;
 	int count = 0;
 
-	if (!p || p == current || p->state == TASK_RUNNING)
+	if (p == current || p->state == TASK_RUNNING)
 		return 0;
 
 	if (!try_get_task_stack(p))