Implementation of AVX2 optimized Fletcher-4

New functionality:
- Preserves existing scalar implementation.
- Adds AVX2 optimized Fletcher-4 computation.
- Fastest routines selected on module load (benchmark).
- Test case for Fletcher-4 added to ztest.

New zcommon module parameters:
-  zfs_fletcher_4_impl (str): selects the implementation to use.
    "fastest" - use the fastest version available
    "cycle"   - cycle trough all available impl for ztest
    "scalar"  - use the original version
    "avx2"    - new AVX2 implementation if available

Performance comparison (Intel i7 CPU, 1MB data buffers):
- Scalar:  4216 MB/s
- AVX2:   14499 MB/s

See contents of `/sys/module/zcommon/parameters/zfs_fletcher_4_impl`
to get 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: Jinshan Xiong <jinshan.xiong@intel.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4330

man/man5/zfs-module-parameters.5

@@ -830,6 +830,23 @@ Start syncing out a transaction group if there is at least this much dirty data.
 Default value: \fB67,108,864\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_fletcher_4_impl\fR (string)
+.ad
+.RS 12n
+Select a fletcher 4 implementation.
+.sp
+Supported selectors are: \fBfastest\fR, \fBscalar\fR, and \fBavx2\fR when
+AVX2 is supported by the processor.  If multiple implementations of fletcher 4
+are available the \fBfastest\fR will be chosen using a micro benchmark.
+Selecting \fBscalar\fR results in the original CPU based calculation being
+used, \fBavx2\fR uses the AVX2 vector instructions to compute a fletcher 4.
+.sp
+Default value: \fBfastest\fR.
+.RE
+
 .sp
 .ne 2
 .na