Fix error message in BPFMap.Update (#29)

When trying to obtain an unsafe pointer to the value type, upon failure
the code was incorrectly returning an error message which contained the
type of the key, not the value. This patch fixes it so that the correct
type is returned for the value in the error message.

Additionally this commit contains a small refactor which uses a type
switch rather than if-else statements for readability.

libbpfgo.go

@@ -195,8 +195,8 @@ import "C"
 
 import (
 	"fmt"
-	"path/filepath"
 	"net"
+	"path/filepath"
 	"strings"
 	"sync"
 	"syscall"
@@ -442,21 +442,22 @@ func (b *BPFMap) GetMaxEntries() uint32 {
 
 func GetUnsafePointer(data interface{}) (unsafe.Pointer, error) {
 	var dataPtr unsafe.Pointer
-	if k, isType := data.(int8); isType {
+	switch k := data.(type) {
+	case int8:
 		dataPtr = unsafe.Pointer(&k)
-	} else if k, isType := data.(uint8); isType {
+	case uint8:
 		dataPtr = unsafe.Pointer(&k)
-	} else if k, isType := data.(int32); isType {
+	case int32:
 		dataPtr = unsafe.Pointer(&k)
-	} else if k, isType := data.(uint32); isType {
+	case uint32:
 		dataPtr = unsafe.Pointer(&k)
-	} else if k, isType := data.(int64); isType {
+	case int64:
 		dataPtr = unsafe.Pointer(&k)
-	} else if k, isType := data.(uint64); isType {
+	case uint64:
 		dataPtr = unsafe.Pointer(&k)
-	} else if k, isType := data.([]byte); isType {
+	case []byte:
 		dataPtr = unsafe.Pointer(&k[0])
-	} else {
+	default:
 		return nil, fmt.Errorf("unknown data type %T", data)
 	}
 
@@ -507,7 +508,7 @@ func (b *BPFMap) Update(key, value interface{}) error {
 	}
 	valuePtr, err := GetUnsafePointer(value)
 	if err != nil {
-		return fmt.Errorf("failed to update map %s: unknown value type %T", b.name, key)
+		return fmt.Errorf("failed to update map %s: unknown value type %T", b.name, value)
 	}
 
 	errC := C.bpf_map_update_elem(b.fd, keyPtr, valuePtr, C.BPF_ANY)