locking/mutex: Remove repeated declaration

Commit 0cd39f4600ed ("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 <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Acked-by: Waiman Long <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

include/linux/mutex.h

···
 #include <linux/osq_lock.h>
 #include <linux/debug_locks.h>
 
+struct ww_class;
 struct ww_acquire_ctx;
 
 /*
···
 	struct lockdep_map	dep_map;
 #endif
 };
-
-struct ww_class;
-struct ww_acquire_ctx;
 
 struct ww_mutex {
 	struct mutex base;