SIMD implementation of vdev_raidz generate and reconstruct routines

This is a new implementation of RAIDZ1/2/3 routines using x86_64
scalar, SSE, and AVX2 instruction sets. Included are 3 parity
generation routines (P, PQ, and PQR) and 7 reconstruction routines,
for all RAIDZ level. On module load, a quick benchmark of supported
routines will select the fastest for each operation and they will
be used at runtime. Original implementation is still present and
can be selected via module parameter.

Patch contains:
- specialized gen/rec routines for all RAIDZ levels,
- new scalar raidz implementation (unrolled),
- two x86_64 SIMD implementations (SSE and AVX2 instructions sets),
- fastest routines selected on module load (benchmark).
- cmd/raidz_test - verify and benchmark all implementations
- added raidz_test to the ZFS Test Suite

New zfs module parameters:
- zfs_vdev_raidz_impl (str): selects the implementation to use. On
  module load, the parameter will only accept first 3 options, and
  the other implementations can be set once module is finished
  loading. Possible values for this option are:
    "fastest" - use the fastest math available
    "original" - use the original raidz code
    "scalar" - new scalar impl
    "sse" - new SSE impl if available
    "avx2" - new AVX2 impl if available

See contents of `/sys/module/zfs/parameters/zfs_vdev_raidz_impl` to
get the list of supported values. If an implementation is not supported
on the system, it will not be shown. Currently selected option is
enclosed in `[]`.

Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4328

man/man5/zfs-module-parameters.5

@@ -1674,6 +1674,33 @@ Aggregate write I/O over gap
 Default value: \fB4,096\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_vdev_raidz_impl\fR (string)
+.ad
+.RS 12n
+Parameter for selecting raidz implementation to use.
+
+Options marked (always) below may be selected on module load as they are
+supported on all systems.
+The remaining options may only be set after the module is loaded, as they
+are available only if the implementations are compiled in and supported
+on the running system.
+
+Once the module is loaded, the content of
+/sys/module/zfs/parameters/zfs_vdev_raidz_impl will show available options
+with the currently selected one enclosed in [].
+Possible options are:
+  fastest  - (always) implementation selected using built-in benchmark
+  original - (always) original raidz implementation
+  scalar   - (always) scalar raidz implementation
+  sse      - implementation using SSE instruction set (64bit x86 only)
+  avx2     - implementation using AVX2 instruction set (64bit x86 only)
+.sp
+Default value: \fBfastest\fR.
+.RE
+
 .sp
 .ne 2
 .na
@@ -1739,7 +1766,7 @@ Default value: \fB1,048,576\fR.
 .ad
 .RS 12n
 A zevent will be logged if a ZIO operation takes more than N milliseconds to
-complete. Note that this is only a logging facility, not a timeout on 
+complete. Note that this is only a logging facility, not a timeout on
 operations.
 .sp
 Default value: \fB30,000\fR.