lockref: include mutex.h rather than reinvent arch_mutex_cpu_relax

arch_mutex_cpu_relax is already conditionally defined in mutex.h, so
simply include that header rather than replicate the code here.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/lockref.c

 #include <linux/export.h>
 #include <linux/lockref.h>
+#include <linux/mutex.h>
 
 #if USE_CMPXCHG_LOCKREF
 
@@ -11,14 +12,6 @@
 # define cmpxchg64_relaxed cmpxchg64
 #endif
 
-/*
- * Allow architectures to override the default cpu_relax() within CMPXCHG_LOOP.
- * This is useful for architectures with an expensive cpu_relax().
- */
-#ifndef arch_mutex_cpu_relax
-# define arch_mutex_cpu_relax() cpu_relax()
-#endif
-
 /*
  * Note that the "cmpxchg()" reloads the "old" value for the
  * failure case.