9p: Replace the fidlist with an IDR

The p9_idpool being used to allocate the IDs uses an IDR to allocate
the IDs ... which we then keep in a doubly-linked list, rather than in
the IDR which allocated them.  We can use an IDR directly which saves
two pointers per p9_fid, and a tiny memory allocation per p9_client.

Link: http://lkml.kernel.org/r/20180711210225.19730-4-willy@infradead.org
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>

include/net/9p/client.h

@@ -27,6 +27,7 @@
 #define NET_9P_CLIENT_H
 
 #include <linux/utsname.h>
+#include <linux/idr.h>
 
 /* Number of requests per row */
 #define P9_ROW_MAXTAG 255
@@ -128,8 +129,7 @@ struct p9_req_t {
  * @proto_version: 9P protocol version to use
  * @trans_mod: module API instantiated with this client
  * @trans: tranport instance state and API
- * @fidpool: fid handle accounting for session
- * @fidlist: List of active fid handles
+ * @fids: All active FID handles
  * @tagpool - transaction id accounting for session
  * @reqs - 2D array of requests
  * @max_tag - current maximum tag id allocated
@@ -169,8 +169,7 @@ struct p9_client {
 		} tcp;
 	} trans_opts;
 
-	struct p9_idpool *fidpool;
-	struct list_head fidlist;
+	struct idr fids;
 
 	struct p9_idpool *tagpool;
 	struct p9_req_t *reqs[P9_ROW_MAXTAG];
@@ -188,7 +187,6 @@ struct p9_client {
  * @iounit: the server reported maximum transaction size for this file
  * @uid: the numeric uid of the local user who owns this handle
  * @rdir: readdir accounting structure (allocated on demand)
- * @flist: per-client-instance fid tracking
  * @dlist: per-dentry fid tracking
  *
  * TODO: This needs lots of explanation.
@@ -204,7 +202,6 @@ struct p9_fid {
 
 	void *rdir;
 
-	struct list_head flist;
 	struct hlist_node dlist;	/* list of all fids attached to a dentry */
 };