nfs: stop using writeback internals for WB_WRITEBACK accounting

Convert NFS WB_WRITEBACK accounting to writeback helper, eliminating
direct access to writeback.

Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kundan Kumar <kundan.kumar@samsung.com>
Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Link: https://patch.msgid.link/20260213054634.79785-5-kundan.kumar@samsung.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Kundan Kumar

fs/nfs/internal.h

@@ -859,7 +859,7 @@ static inline void nfs_folio_mark_unstable(struct folio *folio,
 		 * writeback is happening on the server now.
 		 */
 		node_stat_mod_folio(folio, NR_WRITEBACK, nr);
-		wb_stat_mod(&inode_to_bdi(inode)->wb, WB_WRITEBACK, nr);
+		bdi_wb_stat_mod(inode, WB_WRITEBACK, nr);
 		__mark_inode_dirty(inode, I_DIRTY_DATASYNC);
 	}
 }

fs/nfs/write.c

@@ -872,8 +872,7 @@ static void nfs_folio_clear_commit(struct folio *folio)
 		long nr = folio_nr_pages(folio);
 
 		node_stat_mod_folio(folio, NR_WRITEBACK, -nr);
-		wb_stat_mod(&inode_to_bdi(folio->mapping->host)->wb,
-			    WB_WRITEBACK, -nr);
+		bdi_wb_stat_mod(folio->mapping->host, WB_WRITEBACK, -nr);
 	}
 }