nfs: fix comment and add warn_on for PG_INODE_REF

Fix the comment in nfs_page.h for PG_INODE_REF to reflect that it's no longer
set only on head requests. Also add a WARN_ON_ONCE in nfs_inode_remove_request
as PG_INODE_REF should always be set.
Suggested-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Weston Andros Adamson <dros@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/write.c

@@ -707,6 +707,8 @@ static void nfs_inode_remove_request(struct nfs_page *req)
 
 	if (test_and_clear_bit(PG_INODE_REF, &req->wb_flags))
 		nfs_release_request(req);
+	else
+		WARN_ON_ONCE(1);
 }
 
 static void

include/linux/nfs_page.h

@@ -26,7 +26,7 @@ enum {
 	PG_MAPPED,		/* page private set for buffered io */
 	PG_CLEAN,		/* write succeeded */
 	PG_COMMIT_TO_DS,	/* used by pnfs layouts */
-	PG_INODE_REF,		/* extra ref held by inode (head req only) */
+	PG_INODE_REF,		/* extra ref held by inode when in writeback */
 	PG_HEADLOCK,		/* page group lock of wb_head */
 	PG_TEARDOWN,		/* page group sync for destroy */
 	PG_UNLOCKPAGE,		/* page group sync bit in read path */