NFS: Add a count of the number of unstable writes carried by an inode

In order to know when we should do opportunistic commits of the unstable
writes, when the VM is doing a background flush, we add a field to count
the number of unstable writes.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/nfs_fs.h

@@ -166,6 +166,7 @@ struct nfs_inode {
 	struct radix_tree_root	nfs_page_tree;
 
 	unsigned long		npages;
+	unsigned long		ncommit;
 
 	/* Open contexts for shared mmap writes */
 	struct list_head	open_files;