new inode method: ->free_inode()

A lot of ->destroy_inode() instances end with call_rcu() of a callback
that does RCU-delayed part of freeing.  Introduce a new method for
doing just that, with saner signature.

Rules:
->destroy_inode		->free_inode
	f			g		immediate call of f(),
						RCU-delayed call of g()
	f			NULL		immediate call of f(),
						no RCU-delayed calls
	NULL			g		RCU-delayed call of g()
	NULL			NULL		RCU-delayed default freeing

IOW, NULL ->free_inode gives the same behaviour as now.

Note that NULL, NULL is equivalent to NULL, free_inode_nonrcu; we could
mandate the latter form, but that would have very little benefit beyond
making rules a bit more symmetric.  It would break backwards compatibility,
require extra boilerplate and expected semantics for (NULL, NULL) pair
would have no use whatsoever...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -694,7 +694,10 @@ struct inode {
 #ifdef CONFIG_IMA
 	atomic_t		i_readcount; /* struct files open RO */
 #endif
-	const struct file_operations	*i_fop;	/* former ->i_op->default_file_ops */
+	union {
+		const struct file_operations	*i_fop;	/* former ->i_op->default_file_ops */
+		void (*free_inode)(struct inode *);
+	};
 	struct file_lock_context	*i_flctx;
 	struct address_space	i_data;
 	struct list_head	i_devices;
@@ -1903,6 +1906,7 @@ extern loff_t vfs_dedupe_file_range_one(struct file *src_file, loff_t src_pos,
 struct super_operations {
    	struct inode *(*alloc_inode)(struct super_block *sb);
 	void (*destroy_inode)(struct inode *);
+	void (*free_inode)(struct inode *);
 
    	void (*dirty_inode) (struct inode *, int flags);
 	int (*write_inode) (struct inode *, struct writeback_control *wbc);