pNFS: Add infrastructure for cleaning up per-layout commit structures

Ensure that both the file and flexfiles layout types clean up when
freeing the layout segments.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

include/linux/nfs_xdr.h

@@ -1275,6 +1275,7 @@ struct pnfs_commit_array {
 	struct list_head lseg_list;
 	struct pnfs_layout_segment *lseg;
 	struct rcu_head rcu;
+	refcount_t refcount;
 	unsigned int nbuckets;
 	struct pnfs_commit_bucket buckets[];
 };