f2fs: introduce F2FS_I_SB, F2FS_M_SB, and F2FS_P_SB

This patch adds three inline functions to clean up dirty casting codes.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/node.h b/fs/f2fs/node.h
index 8a116a4..b24f588 100644
--- a/fs/f2fs/node.h
+++ b/fs/f2fs/node.h
@@ -197,8 +197,7 @@
 
 static inline void fill_node_footer_blkaddr(struct page *page, block_t blkaddr)
 {
-	struct f2fs_sb_info *sbi = F2FS_SB(page->mapping->host->i_sb);
-	struct f2fs_checkpoint *ckpt = F2FS_CKPT(sbi);
+	struct f2fs_checkpoint *ckpt = F2FS_CKPT(F2FS_P_SB(page));
 	struct f2fs_node *rn = F2FS_NODE(page);
 
 	rn->footer.cp_ver = ckpt->checkpoint_ver;