Revert "Pre-allocate vdev I/O buffers"

Commit 86dd0fd added preallocated I/O buffers.  This is no longer
required after the recent kmem changes designed to make our memory
allocation interfaces behave more like those found on Illumos.  A
deadlock in this situation is no longer possible.

However, these allocations still have the potential to be expensive.
So a potential future optimization might be to perform then KM_NOSLEEP
so that they either succeed of fail quicky.  Either case is acceptable
here because we can safely abort the aggregation.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/sys/vdev_impl.h

@@ -50,7 +50,6 @@ extern "C" {
  * Forward declarations that lots of things need.
  */
 typedef struct vdev_queue vdev_queue_t;
-typedef struct vdev_io vdev_io_t;
 typedef struct vdev_cache vdev_cache_t;
 typedef struct vdev_cache_entry vdev_cache_entry_t;
 
@@ -117,16 +116,10 @@ struct vdev_queue {
 	uint64_t	vq_last_offset;
 	hrtime_t	vq_io_complete_ts; /* time last i/o completed */
 	hrtime_t	vq_io_delta_ts;
-	list_t		vq_io_list;
 	zio_t		vq_io_search; /* used as local for stack reduction */
 	kmutex_t	vq_lock;
 };
 
-struct vdev_io {
-	char		vi_buffer[SPA_MAXBLOCKSIZE]; /* Must be first */
-	list_node_t	vi_node;
-};
-
 /*
  * Virtual device descriptor
  */