cifs: add refcounting to cifs_readdata structures

This isn't strictly necessary for the async readpages code, but the
uncached version will need to be able to collect the replies after
issuing the calls. Add a kref to cifs_readdata and use change the
code to take and put references appropriately.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
diff --git a/fs/cifs/cifsproto.h b/fs/cifs/cifsproto.h
index f309b43..63e91c7 100644
--- a/fs/cifs/cifsproto.h
+++ b/fs/cifs/cifsproto.h
@@ -464,6 +464,7 @@
 
 /* asynchronous read support */
 struct cifs_readdata {
+	struct kref			refcount;
 	struct cifsFileInfo		*cfile;
 	struct address_space		*mapping;
 	__u64				offset;
@@ -478,6 +479,7 @@
 	struct kvec			iov[1];
 };
 
+void cifs_readdata_release(struct kref *refcount);
 int cifs_async_readv(struct cifs_readdata *rdata);
 
 /* asynchronous write support */