fusefrontend_reverse: fix GETATTR panic
We don't implement Getattr(), so don't try to call it.
Reported at https://github.com/rfjakob/gocryptfs/issues/519#issuecomment-718790790 :
15:22:53.414101 rx 3344: READ n2565 {Fh 7 [42143744 +131072) L 0 RDONLY,0x8000}
15:22:53.414274 rx 3342: READ n2565 {Fh 7 [42012672 +131072) L 0 RDONLY,0x8000}
15:22:53.414787 rx 3346: READ n2565 {Fh 7 [42274816 +131072) L 0 RDONLY,0x8000}
15:22:53.414806 tx 3336: OK, 131072b data "\xcb\xd3<\"!-\xa7\xc4"...
15:22:53.414874 rx 3348: GETATTR n1446 {Fh 0}
panic: interface conversion: *fusefrontend_reverse.File is not fs.FileGetattrer: missing method Getattr
goroutine 451 [running]:
github.com/rfjakob/gocryptfs/internal/fusefrontend_reverse.(*Node).Getattr(0xc00034c880, 0x5620579784c0, 0xc000593e60, 0x562057939800, 0xc000218050, 0xc0000fc108, 0x0)
github.com/rfjakob/gocryptfs/internal/fusefrontend_reverse/node.go:69 +0x273
github.com/hanwen/go-fuse/v2/fs.(*rawBridge).getattr(0xc00011e000, 0x5620579784c0, 0xc000593e60, 0xc00034c880, 0x562057939800, 0xc000218050, 0xc0000fc108, 0xbfded1ef58ba7b13)
github.com/hanwen/go-fuse/v2@v2.0.4-0.20200908172753-0b6cbc515082/fs/bridge.go:500 +0x2d4
github.com/hanwen/go-fuse/v2/fs.(*rawBridge).GetAttr(0xc00011e000, 0xc0000e0000, 0xc0000fc198, 0xc0000fc108, 0x0)
github.com/hanwen/go-fuse/v2@v2.0.4-0.20200908172753-0b6cbc515082/fs/bridge.go:488 +0x11c
github.com/hanwen/go-fuse/v2/fuse.doGetAttr(0xc000120000, 0xc0000fc000)
github.com/hanwen/go-fuse/v2@v2.0.4-0.20200908172753-0b6cbc515082/fuse/opcode.go:287 +0x6f
github.com/hanwen/go-fuse/v2/fuse.(*Server).handleRequest(0xc000120000, 0xc0000fc000, 0xc000000000)
github.com/hanwen/go-fuse/v2@v2.0.4-0.20200908172753-0b6cbc515082/fuse/server.go:472 +0x2c1
github.com/hanwen/go-fuse/v2/fuse.(*Server).loop(0xc000120000, 0xc000288001)
github.com/hanwen/go-fuse/v2@v2.0.4-0.20200908172753-0b6cbc515082/fuse/server.go:445 +0x18c
created by github.com/hanwen/go-fuse/v2/fuse.(*Server).readRequest
github.com/hanwen/go-fuse/v2@v2.0.4-0.20200908172753-0b6cbc515082/fuse/server.go:312 +0x419
1c1692c4d9