fuse: avoid panic in debug print routines

Add `isTest` to avoid panicking outside of test scenarios, in case we
mess up the flag definitions. The latter can easily happen, due to
subtle differences between architectures.

Change-Id: I4085de49e39eddf328269a7a3e694818c693e195

fuse/print.go

@@ -12,6 +12,7 @@ import (
 )
 
 var (
+	isTest         bool
 	writeFlagNames = newFlagNames([]flagNameEntry{
 		{WRITE_CACHE, "CACHE"},
 		{WRITE_LOCKOWNER, "LOCKOWNER"},
@@ -111,7 +112,7 @@ func (names *flagNames) set(flag int64, name string) {
 	entry := flagNameEntry{bits: flag, name: name}
 	for i := 0; i < 64; i++ {
 		if flag&(1<<i) != 0 {
-			if ie := names[i]; ie.bits != 0 {
+			if ie := names[i]; ie.bits != 0 && isTest {
 				panic(fmt.Sprintf("%s (%x) overlaps with %s (%x)", name, flag, ie.name, ie.bits))
 			}
 			names[i] = entry

fuse/print_test.go

@@ -8,6 +8,10 @@ import (
 	"testing"
 )
 
+func init() {
+	isTest = true
+}
+
 // verify that flagString always formats flags in the same order.
 func TestFlagStringOrder(t *testing.T) {
 	var flags int64 = CAP_ASYNC_READ | CAP_SPLICE_WRITE | CAP_READDIRPLUS | CAP_MAX_PAGES | CAP_EXPLICIT_INVAL_DATA