block: blk-map: re-use existing 'reading' variable instead of checking direction
block: fix blk_queue_end_tag()

In order to allow blk_queue_resize_tags() to shrink the tag space
blk_queue_end_tag() must always complete tags with a value less than
real_max_depth regardless of the current max_depth.  The comment about
'handling the shrink case' seems to be what prompted changes in this
space, so remove it and BUG on all invalid tags (made even simpler by
Matthew's suggestion to use an unsigned compare).