locking/mutex: Remove repeated declaration

Commit 0cd39f46

 ("locking/seqlock, headers: Untangle the spaghetti monster")
introduces 'struct ww_acquire_ctx' again, remove the repeated declaration and move
the pre-declarations to the top.

Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
Link: https://lore.kernel.org/r/1616564440-61318-1-git-send-email-zhangshaokun@hisilicon.com