[PATCH] blk: fix tag shrinking (revive real_max_size)

My patch in commit fa72b903

 incorrectly
removed blk_queue_tag->real_max_depth.

The original resize implementation was incorrect in the following
points.

 * actual allocation size of tag_index was shorter than real_max_size,
   but assumed to be of the same size, possibly causing memory access
   beyond the allocated area.
 * bits in tag_map between max_deptn and real_max_depth were
   initialized to 1's, making the tags permanently reserved.

In an attempt to fix above two bugs, I had removed allocation optimization
in init_tag_map and real_max_size.  Tag map/index were allocated and freed
immediately during resize.

Unfortunately, I wasn't considering that tag map/index can be resized
dynamically with tags beyond new_depth active.  This led to accessing
freed area after shrinking tags and led to the following bug reporting
thread on linux-scsi.

   http://marc.theaimsgroup.com/?l=linux-scsi&m=112319898111885&w=2

To fix the problem, I've revived real_max_depth without allocation
optimization in init_tag_map, and Andrew Vasquez confirmed that the
problem was fixed.  As Jens is not going to be available for a week, he
asked me to make sure that this patch reaches you.

   http://marc.theaimsgroup.com/?l=linux-scsi&m=112325778530886&w=2

Also, a comment was added to make sure that real_max_size is needed for
dynamic shrinking.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>