illumos gate

In a case a read-only filesystem is shared over NFS, the NFSv2/v3 server does not check hard enough to prevent modifications of such filesystem.

Steps to reproduce this issue (reproducible only without the fix for #4638):

1. create a ZFS filesystem
2. create a file in the filesystem
3. change the owner of the file to nobody
4. set readonly property of the filesystem to on
5. share the filesystem over NFS
6. mount the filesystem on a Windows machine
7. try to write to the file from the Windows machine
8. the write will succeed, but it shouldn't

To fix this issue we need to enhance the rdonly() macro to call vn_is_readonly(), similarly as it is done in the rdonly4().