radix-tree: fix comment about "exceptional" bits

The bottom two bits of radix tree entries are reserved for special use
by the radix tree code itself.  A comment detailing their usage was
added by commit 3bcadd6fa6 ("radix-tree: free up the bottom bit of
exceptional entries for reuse")

This comment states that if the bottom two bits are '11', this means
that this is a locked exceptional entry.

It turns out that this bit combination was never actually used.  Radix
tree locking for DAX was indeed implemented, but it actually used the
third LSB:

  /* We use lowest available exceptional entry bit for locking */
  #define RADIX_DAX_ENTRY_LOCK (1 << RADIX_TREE_EXCEPTIONAL_SHIFT)

This locking code was also made specific to the DAX code instead of
being generally implemented in radix-tree.h.

So, fix the comment.

Link: http://lkml.kernel.org/r/1468997731-2155-1-git-send-email-ross.zwisler@linux.intel.com
Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/radix-tree.h

@@ -35,7 +35,7 @@
  * 00 - data pointer
  * 01 - internal entry
  * 10 - exceptional entry
- * 11 - locked exceptional entry
+ * 11 - this bit combination is currently unused/reserved
  *
  * The internal entry may be a pointer to the next level in the tree, a
  * sibling entry, or an indicator that the entry in this slot has been moved