move uint64s to beginning of struct to ensure they are 64-bit aligned

See https://golang.org/pkg/sync/atomic/#pkg-note-BUG for reference:
"The first word in (...) an allocated struct (...) can be relied up to
be 64-bit aligned"

This fixes an "unaligned 64-bit atomic operation" panic on 32-bit ARM,
(in my case a Raspberry Pi 4, running 32-bit Raspbian). The panic
happens (at least) during the first login after a service (re)start.

changelog/unreleased/fix-unaligned-atomic-operation-on-arm32.md

@@ -0,0 +1,7 @@
+Bugfix: Fixes "unaligned 64-bit atomic operation" panic on 32-bit ARM
+
+sync/cache had uint64s that were not 64-bit aligned causing panics
+on 32-bit systems during atomic access
+
+https://github.com/owncloud/ocis/pull/1888
+https://github.com/owncloud/ocis/issues/1887

ocis-pkg/sync/cache.go

@@ -8,10 +8,11 @@ import (
 
 // Cache is a barebones cache implementation.
 type Cache struct {
+	// capacity and length have to be the first words
+	// in order to be 64-aligned on 32-bit architectures.
+	capacity, length uint64 // access atomically
 	entries  sync.Map
 	pool     sync.Pool
-	capacity uint64
-	length   uint64
 }
 
 // CacheEntry represents an entry on the cache. You can type assert on V.