Remove misguided HAVE_MUTEX_OWNER check

It is just plain unsafe to peek inside in-kernel
mutex structure and make assumptions about what kernel
does with those internal fields like owner.

Kernel is all too happy to stop doing the expected things
like tracing lock owner once you load a tainted module
like spl/zfs that is not GPL.

As such you will get instant assertion failures like this:

  VERIFY3(((*(volatile typeof((&((&zo->zo_lock)->m_mutex))->owner) *)&
      ((&((&zo->zo_lock)->m_mutex))->owner))) == 
     ((void *)0)) failed (ffff88030be28500 == (null))
  PANIC at zfs_onexit.c:104:zfs_onexit_destroy()
  Showing stack for process 3626
  CPU: 0 PID: 3626 Comm: mkfs.lustre Tainted: P OE ------------ 3.10.0-debug #1
  Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
  Call Trace:
  dump_stack+0x19/0x1b
  spl_dumpstack+0x44/0x50 [spl]
  spl_panic+0xbf/0xf0 [spl]
  zfs_onexit_destroy+0x17c/0x280 [zfs]
  zfsdev_release+0x48/0xd0 [zfs]

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Closes #632 
Closes #633

include/sys/mutex.h

@@ -40,10 +40,8 @@ typedef enum {
 typedef struct {
 	struct mutex		m_mutex;
 	spinlock_t		m_lock;	/* used for serializing mutex_exit */
-#ifndef HAVE_MUTEX_OWNER
 	/* only when kernel doesn't have owner */
 	kthread_t		*m_owner;
-#endif
 #ifdef CONFIG_LOCKDEP
 	kmutex_type_t		m_type;
 #endif /* CONFIG_LOCKDEP */
@@ -58,24 +56,16 @@ spl_mutex_set_owner(kmutex_t *mp)
 	 * kernel will handle its owner, so we don't need to do anything if it
 	 * is defined.
 	 */
-#ifndef HAVE_MUTEX_OWNER
 	mp->m_owner = current;
-#endif
 }
 
 static inline void
 spl_mutex_clear_owner(kmutex_t *mp)
 {
-#ifndef HAVE_MUTEX_OWNER
 	mp->m_owner = NULL;
-#endif
 }
 
-#ifdef HAVE_MUTEX_OWNER
 #define	mutex_owner(mp)		(ACCESS_ONCE(MUTEX(mp)->owner))
-#else
-#define	mutex_owner(mp)		(ACCESS_ONCE((mp)->m_owner))
-#endif
 #define	mutex_owned(mp)		(mutex_owner(mp) == current)
 #define	MUTEX_HELD(mp)		mutex_owned(mp)
 #define	MUTEX_NOT_HELD(mp)	(!MUTEX_HELD(mp))