vfs: Fix IO Error opening a file with O_CREATE|O_RDONLY in --vfs-cache-mode not full

Before this fix, opening a file with `O_CREATE|O_RDONLY` caused an IO error to
be returned when using `--vfs-cache-mode off` or `--vfs-cache-mode writes`.

This was because the file was opened with read intent, but the `O_CREATE`
implies write intent to create the file even though the file is opened
`O_RDONLY`.

This fix sets write intent for the file if `O_CREATE` is passed in which fixes
the problem for all the VFS cache modes.

It also extends the exhaustive open flags testing to `--vfs-cache-mode writes`
as well as `--vfs-cache-mode full` which would have caught this problem.

See: https://forum.rclone.org/t/i-o-error-trashing-file-on-sftp-mount/34317/

vfs/file.go

@@ -698,6 +698,11 @@ func (f *File) Open(flags int) (fd Handle, err error) {
 		write = true
 	}
 
+	// If create is set then set write to force openRW
+	if flags&os.O_CREATE != 0 {
+		write = true
+	}
+
 	// Open the correct sort of handle
 	f.mu.RLock()
 	d := f.d

vfs/read_write_test.go

@@ -642,15 +642,19 @@ func testRWFileHandleOpenTest(t *testing.T, vfs *VFS, test *openTest) {
 }
 
 func TestRWFileHandleOpenTests(t *testing.T) {
-	opt := vfscommon.DefaultOpt
-	opt.CacheMode = vfscommon.CacheModeFull
-	opt.WriteBack = writeBackDelay
-	_, vfs, cleanup := newTestVFSOpt(t, &opt)
-	defer cleanup()
+	for _, cacheMode := range []vfscommon.CacheMode{vfscommon.CacheModeWrites, vfscommon.CacheModeFull} {
+		t.Run(cacheMode.String(), func(t *testing.T) {
+			opt := vfscommon.DefaultOpt
+			opt.CacheMode = cacheMode
+			opt.WriteBack = writeBackDelay
+			_, vfs, cleanup := newTestVFSOpt(t, &opt)
+			defer cleanup()
 
-	for _, test := range openTests {
-		t.Run(test.what, func(t *testing.T) {
-			testRWFileHandleOpenTest(t, vfs, &test)
+			for _, test := range openTests {
+				t.Run(test.what, func(t *testing.T) {
+					testRWFileHandleOpenTest(t, vfs, &test)
+				})
+			}
 		})
 	}
 }