libfs: add support for special files not backed by a filesystem

lib/libfs/file.c

@@ -1,5 +1,10 @@
 #include "file.h"
 
+void fs_file_set_ops(struct fs_file *f, const struct fs_file_ops *ops)
+{
+	f->f_ops = ops;
+}
+
 struct fs_inode *fs_file_get_inode(const struct fs_file *f)
 {
 	return f->f_inode;

lib/libfs/include/fs/file.h

@@ -23,6 +23,7 @@ struct fs_file_ops {
 		*f_readdir)(struct fs_file *, struct xpc_buffer *, off_t *);
 };
 
+extern void fs_file_set_ops(struct fs_file *f, const struct fs_file_ops *ops);
 extern struct fs_inode *fs_file_get_inode(const struct fs_file *f);
 extern size_t fs_file_get_cursor(const struct fs_file *f);
 extern enum fs_status fs_file_read(

lib/libfs/interface/seek.c

@@ -39,7 +39,7 @@ extern kern_status_t fs_msg_seek(
 		return KERN_OK;
 	}
 
-	if (new_offset > f->f_inode->i_size) {
+	if (f->f_inode && new_offset > f->f_inode->i_size) {
 		*out_err = EINVAL;
 		return KERN_OK;
 	}