fuse: Return EPERM rather than ENOSYS from link()

link() is documented to return EPERM when a filesystem doesn't support
the operation, return that instead.

Link: https://github.com/libfuse/libfuse/issues/925
Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/dir.c

@@ -1137,6 +1137,8 @@ static int fuse_link(struct dentry *entry, struct inode *newdir,
 	else if (err == -EINTR)
 		fuse_invalidate_attr(inode);
 
+	if (err == -ENOSYS)
+		err = -EPERM;
 	return err;
 }