index: hash blob IDs with maphash.Bytes (#22070)

Avoid constructing a maphash.Hash on every lookup and insert.

internal/repository/index/indexmap.go

@@ -31,7 +31,7 @@ type indexMap struct {
 	buckets    []uint
 	numentries uint
 
-	mh maphash.Hash
+	seed maphash.Seed
 
 	blockList hashedArrayTree
 }
@@ -176,15 +176,13 @@ func (m *indexMap) hash(id restic.ID) uint {
 	// While SHA-256 should be collision-resistant, for hash table indices
 	// we use only a few bits of it and finding collisions for those is
 	// much easier than breaking the whole algorithm.
-	mh := maphash.Hash{}
-	mh.SetSeed(m.mh.Seed())
-	_, _ = mh.Write(id[:])
-	h := uint(mh.Sum64())
+	h := uint(maphash.Bytes(m.seed, id[:]))
 	return h & uint(len(m.buckets)-1)
 }
 
 func (m *indexMap) init() {
 	const initialBuckets = 64
+	m.seed = maphash.MakeSeed()
 	m.buckets = make([]uint, initialBuckets)
 	// first entry in blockList serves as null byte
 	m.blockList = *newHAT()