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>

include/net/9p/9p.h

@@ -38,6 +38,8 @@
  * @P9_DEBUG_SLABS: memory management tracing
  * @P9_DEBUG_FCALL: verbose dump of protocol messages
  * @P9_DEBUG_FID: fid allocation/deallocation tracking
+ * @P9_DEBUG_PKT: packet marshalling/unmarshalling
+ * @P9_DEBUG_FSC: FS-cache tracing
  *
  * These flags are passed at mount time to turn on various levels of
  * verbosity and tracing which will be output to the system logs.
@@ -54,6 +56,7 @@ enum p9_debug_flags {
 	P9_DEBUG_FCALL =	(1<<8),
 	P9_DEBUG_FID =		(1<<9),
 	P9_DEBUG_PKT =		(1<<10),
+	P9_DEBUG_FSC =		(1<<11),
 };
 
 #ifdef CONFIG_NET_9P_DEBUG