ui/progress: Use mutex instead of atomic

The counter value needs to be aligned to 64 bit in memory for the
atomic functions to work on some platform (such as 32 bit ARM).

The atomic package says in its documentation:

> These functions require great care to be used correctly. Except for
> special, low-level applications, synchronization is better done with
> channels or the facilities of the sync package.

This commit replaces the atomic functions with a simple sync.Mutex, so
we don't have to care about alignment.

internal/ui/progress/counter.go

@@ -3,7 +3,6 @@ package progress
 import (
 	"os"
 	"sync"
-	"sync/atomic"
 	"time"
 
 	"github.com/restic/restic/internal/debug"
@@ -25,7 +24,9 @@ type Counter struct {
 	stopped chan struct{} // Closed by run.
 	stop    chan struct{} // Close to stop run.
 	tick    *time.Ticker
-	value   uint64
+
+	valueMutex sync.Mutex
+	value      uint64
 }
 
 // New starts a new Counter.
@@ -52,7 +53,10 @@ func (c *Counter) Add(v uint64) {
 	if c == nil {
 		return
 	}
-	atomic.AddUint64(&c.value, v)
+
+	c.valueMutex.Lock()
+	c.value += v
+	c.valueMutex.Unlock()
 }
 
 // Done tells a Counter to stop and waits for it to report its final value.
@@ -66,7 +70,13 @@ func (c *Counter) Done() {
 	*c = Counter{} // Prevent reuse.
 }
 
-func (c *Counter) get() uint64 { return atomic.LoadUint64(&c.value) }
+func (c *Counter) get() uint64 {
+	c.valueMutex.Lock()
+	v := c.value
+	c.valueMutex.Unlock()
+
+	return v
+}
 
 func (c *Counter) run() {
 	defer close(c.stopped)