MFV r287624: 5987 zfs prefetch code needs work

MFV r287624: 5987 zfs prefetch code needs work

Rewrite the ZFS prefetch code to detect only forward, sequential
streams.

The following kstats have been added:

    kstat.zfs.misc.arcstats.sync_wait_for_async

	How many sync reads have waited for async read
	to complete. (less is better)

    kstat.zfs.misc.arcstats.demand_hit_predictive_prefetch

	How many demand read didn't have to wait for I/O
	because of predictive prefetch.  (more is better)

zfetch kstats have been similified to hits, misses, and max_streams,
with max_streams representing times when we were not able to create
new stream because we already have the maximum number of sequences
for a file.

The sysctl variable/loader tunable vfs.zfs.zfetch.block_cap have been
replaced by vfs.zfs.zfetch.max_distance, which controls maximum bytes
to prefetch per stream.

illumos/illumos-gate@cf6106c8a0d6598b045811f9650d66e07eb332af

Illumos ZFS issues:

    5987 zfs prefetch code needs work
    https://www.illumos.org/issues/5987