9p: Add fscache support to 9p

This patch adds a persistent, read-only caching facility for
9p clients using the FS-Cache caching backend.

When the fscache facility is enabled, each inode is associated
with a corresponding vcookie which is an index into the FS-Cache
indexing tree. The FS-Cache indexing tree is indexed at 3 levels:
- session object associated with each mount.
- inode/vcookie
- actual data (pages)

A cache tag is chosen randomly for each session. These tags can
be read off /sys/fs/9p/caches and can be passed as a mount-time
parameter to re-attach to the specified caching session.

Signed-off-by: Abhishek Kulkarni <adkulkar@umail.iu.edu>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

fs/9p/vfs_super.c

@@ -44,20 +44,8 @@
 #include "v9fs_vfs.h"
 #include "fid.h"
 
-static void v9fs_clear_inode(struct inode *);
 static const struct super_operations v9fs_super_ops;
 
-/**
- * v9fs_clear_inode - release an inode
- * @inode: inode to release
- *
- */
-
-static void v9fs_clear_inode(struct inode *inode)
-{
-	filemap_fdatawrite(inode->i_mapping);
-}
-
 /**
  * v9fs_set_super - set the superblock
  * @s: super block
@@ -220,6 +208,10 @@ v9fs_umount_begin(struct super_block *sb)
 }
 
 static const struct super_operations v9fs_super_ops = {
+#ifdef CONFIG_9P_FSCACHE
+	.alloc_inode = v9fs_alloc_inode,
+	.destroy_inode = v9fs_destroy_inode,
+#endif
 	.statfs = simple_statfs,
 	.clear_inode = v9fs_clear_inode,
 	.show_options = generic_show_options,