union: fix panic due to misalignment of struct field in 32 bit architectures

`FS.cacheExpiry` is accessed through sync/atomic.
According to the documentation, "On ARM, 386, and 32-bit MIPS, it is
the caller's responsibility to arrange for 64-bit alignment of 64-bit
words accessed atomically. The first word in a variable or in an
allocated struct, array, or slice can be relied upon to be 64-bit
aligned."
Before commit 1d2fe0d856 this field was
aligned, but then a new field was added to the structure, causing the
test suite to panic on linux/386.
No other field is used with sync/atomic, so `cacheExpiry` can just be
placed at the beginning of the stuct to ensure it is always aligned.

backend/union/upstream/upstream.go

@@ -24,6 +24,10 @@ var (
 
 // Fs is a wrap of any fs and its configs
 type Fs struct {
+	// In order to ensure memory alignment on 32-bit architectures
+	// when this field is accessed through sync/atomic functions,
+	// it must be the first entry in the struct
+	cacheExpiry int64 // usage cache expiry time
 	fs.Fs
 	RootFs      fs.Fs
 	RootPath    string
@@ -32,7 +36,6 @@ type Fs struct {
 	creatable   bool
 	usage       *fs.Usage     // Cache the usage
 	cacheTime   time.Duration // cache duration
-	cacheExpiry int64         // usage cache expiry time
 	cacheMutex  sync.RWMutex
 	cacheOnce   sync.Once
 	cacheUpdate bool // if the cache is updating