OpenZFS 6322 - ZFS indirect block predictive prefetch

OpenZFS 6322 - ZFS indirect block predictive prefetch

For quite some time I was thinking about possibility to prefetch
ZFS indirection tables while doing sequential reads or writes.
Recent changes in predictive prefetcher made that much easier to
do. My tests on zvol with 16KB block size on 5x striped and 2x
mirrored pool of 10 disks show almost double throughput on sequential
read, and almost tripple on sequential rewrite. While for read alike
effect can be received from increasing maximal prefetch distance
(though at higher memory cost), for rewrite there is no other
solution so far.

Authored by: Alexander Motin <mav@freebsd.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Ported-by: kernelOfTruth kerneloftruth@gmail.com
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/6322
OpenZFS-commit: https://github.com/illumos/illumos-gate/commit/cb92f413
Closes #5040

Porting notes:
- Change from upstream in module/zfs/dbuf.c in 'int dbuf_read' due
  to commit 5f6d0b6 'Handle block pointers with a corrupt logical size'

- Difference from upstream in module/zfs/dmu_zfetch.c,
  uint32_t zfetch_max_idistance -> unsigned int zfetch_max_idistance

- Variables have been initialized at the beginning of the function
 (void dmu_zfetch) to resemble the order of occurrence and account
 for C99, C11 mode errors.