bit_spinlock: don't play preemption games inside the busy loop

When we are waiting for the bit-lock to be released, and are looping
over the 'cpu_relax()' should not be doing anything else - otherwise we
miss the point of trying to do the whole 'cpu_relax()'.

Do the preemption enable/disable around the loop, rather than inside of
it.

Noticed when I was looking at the code generation for the dcache
__d_drop usage, and the code just looked very odd.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/bit_spinlock.h

@@ -23,12 +23,12 @@ static inline void bit_spin_lock(int bitnum, unsigned long *addr)
 	preempt_disable();
 #if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
 	while (unlikely(test_and_set_bit_lock(bitnum, addr))) {
-		while (test_bit(bitnum, addr)) {
 		preempt_enable();
+		do {
 			cpu_relax();
+		} while (test_bit(bitnum, addr));
 		preempt_disable();
 	}
-	}
 #endif
 	__acquire(bitlock);
 }