9p: Use a slab for allocating requests

Replace the custom batch allocation with a slab.  Use an IDR to store
pointers to the active requests instead of an array.  We don't try to
handle P9_NOTAG specially; the IDR will happily shrink all the way back
once the TVERSION call has completed.

Link: http://lkml.kernel.org/r/20180711210225.19730-6-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>
diff --git a/net/9p/mod.c b/net/9p/mod.c
index 253ba82..0da56d6 100644
--- a/net/9p/mod.c
+++ b/net/9p/mod.c
@@ -171,11 +171,17 @@ void v9fs_put_trans(struct p9_trans_module *m)
  */
 static int __init init_p9(void)
 {
+	int ret;
+
+	ret = p9_client_init();
+	if (ret)
+		return ret;
+
 	p9_error_init();
 	pr_info("Installing 9P2000 support\n");
 	p9_trans_fd_init();
 
-	return 0;
+	return ret;
 }
 
 /**
@@ -188,6 +194,7 @@ static void __exit exit_p9(void)
 	pr_info("Unloading 9P2000 support\n");
 
 	p9_trans_fd_exit();
+	p9_client_exit();
 }
 
 module_init(init_p9)