fix: return MaxInt on indexFromInstanceSuffix edge-cases to fix 32bit arch compatibility

[salvacorts]

Loki is failing to build for the linux/arm platform with (see):

59.88 # github.com/grafana/dskit/ring
59.88 vendor/github.com/grafana/dskit/ring/multi_partition_instance_ring.go:151:10: cannot use math.MaxInt64 (untyped int constant 9223372036854775807) as int value in return statement (overflows)
59.88 vendor/github.com/grafana/dskit/ring/multi_partition_instance_ring.go:155:10: cannot use math.MaxInt64 (untyped int constant 9223372036854775807) as int value in return statement (overflows)

linux/arm is a 32bits platform so int is 32 bits, but indexFromInstanceSuffix returns math.MaxInt64 instead of math.MaxInt.

What this PR does:

Which issue(s) this PR fixes:

Fixes PR - #725

Checklist

• Tests updated

[fcjack]

lgtm