ipc,msg: drop dst nil validation in copy_msg

d0edd852 ("ipc: convert invalid scenarios to use WARN_ON") relaxed the
nil dst parameter check, originally being a full BUG_ON.  However, this
check seems quite unnecessary when the only purpose is for
ceckpoint/restore (MSG_COPY flag):

o The copy variable is set initially to nil, apparently as a way of
  ensuring that prepare_copy is previously called.  Which is in fact done,
  unconditionally at the beginning of do_msgrcv.

o There is no concurrency with 'copy' (stack allocated in do_msgrcv).

Furthermore, any errors in 'copy' (and thus prepare_copy/copy_msg) should
always handled by IS_ERR() family.  Therefore remove this check altogether
as it can never occur with the current users.
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/msgutil.c

@@ -123,7 +123,6 @@ struct msg_msg *copy_msg(struct msg_msg *src, struct msg_msg *dst)
 	size_t len = src->m_ts;
 	size_t alen;
 
-	WARN_ON(dst == NULL);
 	if (src->m_ts > dst->m_ts)
 		return ERR_PTR(-EINVAL);