Add new kstat KSTAT_TYPE_READ type

Add a new kstat type for tracking useful statistics about read.
The new KSTAT_TYPE_READ type can be used to track the following
statistics for each read:

 * A unique identifier (uint64_t)
 * The time the entry was added to the list (hrtime_t)
   (*not* wall clock time; relative to the other entries on the list)
 * The objset ID (uint64_t)
 * The object number (uint64_t)
 * The indirection level (uint64_t)
 * The block ID (uint64_t)
 * The name of the function originating the arc_read call (char[24])
 * The arc_flags from the arc_read call (uint32_t)

Signed-off-by: Prakash Surya <surya1@llnl.gov>