locking/qspinlock: Include linux/prefetch.h

kernel/locking/qspinlock.c makes use of prefetchw() but hasn't included
linux/prefetch.h up until now, instead relying upon indirect inclusion
of some header that defines prefetchw().

In the case of MIPS we generally obtain a definition of prefetchw() from
asm/processor.h, included by way of linux/mutex.h, but only for
configurations which select CONFIG_CPU_HAS_PREFETCH. For configurations
which don't this leaves prefetchw() undefined leading to the following
build failure:

  CC      kernel/locking/qspinlock.o
  kernel/locking/qspinlock.c: In function ‘queued_spin_lock_slowpath’:
  kernel/locking/qspinlock.c:549:4: error: implicit declaration of
      function ‘prefetchw’ [-Werror=implicit-function-declaration]
    prefetchw(next);
    ^~~~~~~~~

Fix this by including linux/prefetch.h in order to ensure that we get a
definition of prefetchw().

Signed-off-by: Paul Burton <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Ralf Baechle <[email protected]>

Author: paulburton

kernel/locking/qspinlock.c

@@ -28,6 +28,7 @@
 #include <linux/percpu.h>
 #include <linux/hardirq.h>
 #include <linux/mutex.h>
+#include <linux/prefetch.h>
 #include <asm/byteorder.h>
 #include <asm/qspinlock.h>