mapdir: use EOPNOTSUPP for xattr

[ktock]

Currently (unimplemented) 9pfs's xattr returns ENOTSUP but ls (GNU coreutils) prints error message for this errno. This commit fixes this by using EOPNOTSUPP instead, for supressing the error message.

$ wasmtime --mapdir /test/dir/share::/tmp/share/ /app/out.wasm ls -al /test/dir/share/
ls: /test/dir/share/: Unknown error 524
ls: /test/dir/share/.: Unknown error 524
ls: /test/dir/share/a: Unknown error 524
total 0
d--------- 1 root root  0 Jan  1  1970 .
drwxr-xr-x 3 root root 60 Mar  7 09:45 ..
---------- 1 root root  3 Mar  7 09:05 a