Prevent unnecessary verification of parity blocks while reading (#4683)

* Prevent unnecessary verification of parity blocks while reading erasure
  coded file.
* Update klauspost/reedsolomon and just only reconstruct data blocks while
  reading (prevent unnecessary parity block reconstruction)
* Remove Verification of (all) reconstructed Data and Parity blocks since
  in our case we are protected by bit rot protection. And even if the
  verification would fail (essentially impossible) there is no way to
  definitively say whether the data is still correct or not, so this call
  make no sense for our use case.

vendor/github.com/klauspost/reedsolomon/options.go

@@ -0,0 +1,78 @@
+package reedsolomon
+
+import (
+	"runtime"
+
+	"github.com/klauspost/cpuid"
+)
+
+// Option allows to override processing parameters.
+type Option func(*options)
+
+type options struct {
+	maxGoroutines     int
+	minSplitSize      int
+	useAVX2, useSSSE3 bool
+	usePAR1Matrix     bool
+}
+
+var defaultOptions = options{
+	maxGoroutines: 50,
+	minSplitSize:  512,
+}
+
+func init() {
+	if runtime.GOMAXPROCS(0) <= 1 {
+		defaultOptions.maxGoroutines = 1
+	}
+	// Detect CPU capabilities.
+	defaultOptions.useSSSE3 = cpuid.CPU.SSSE3()
+	defaultOptions.useAVX2 = cpuid.CPU.AVX2()
+}
+
+// WithMaxGoroutines is the maximum number of goroutines number for encoding & decoding.
+// Jobs will be split into this many parts, unless each goroutine would have to process
+// less than minSplitSize bytes (set with WithMinSplitSize).
+// For the best speed, keep this well above the GOMAXPROCS number for more fine grained
+// scheduling.
+// If n <= 0, it is ignored.
+func WithMaxGoroutines(n int) Option {
+	return func(o *options) {
+		if n > 0 {
+			o.maxGoroutines = n
+		}
+	}
+}
+
+// WithMinSplitSize is the minimum encoding size in bytes per goroutine.
+// See WithMaxGoroutines on how jobs are split.
+// If n <= 0, it is ignored.
+func WithMinSplitSize(n int) Option {
+	return func(o *options) {
+		if n > 0 {
+			o.minSplitSize = n
+		}
+	}
+}
+
+func withSSE3(enabled bool) Option {
+	return func(o *options) {
+		o.useSSSE3 = enabled
+	}
+}
+
+func withAVX2(enabled bool) Option {
+	return func(o *options) {
+		o.useAVX2 = enabled
+	}
+}
+
+// WithPAR1Matrix causes the encoder to build the matrix how PARv1
+// does. Note that the method they use is buggy, and may lead to cases
+// where recovery is impossible, even if there are enough parity
+// shards.
+func WithPAR1Matrix() Option {
+	return func(o *options) {
+		o.usePAR1Matrix = true
+	}
+}