f2fs: add help function META_MAPPING

Introduce help function META_MAPPING() to get the cache meta blocks'
address space.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index 4de0345..f9d4f7d 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -30,7 +30,7 @@
  */
 struct page *grab_meta_page(struct f2fs_sb_info *sbi, pgoff_t index)
 {
-	struct address_space *mapping = sbi->meta_inode->i_mapping;
+	struct address_space *mapping = META_MAPPING(sbi);
 	struct page *page = NULL;
 repeat:
 	page = grab_cache_page(mapping, index);
@@ -50,7 +50,7 @@
  */
 struct page *get_meta_page(struct f2fs_sb_info *sbi, pgoff_t index)
 {
-	struct address_space *mapping = sbi->meta_inode->i_mapping;
+	struct address_space *mapping = META_MAPPING(sbi);
 	struct page *page;
 repeat:
 	page = grab_cache_page(mapping, index);
@@ -128,7 +128,7 @@
 long sync_meta_pages(struct f2fs_sb_info *sbi, enum page_type type,
 						long nr_to_write)
 {
-	struct address_space *mapping = sbi->meta_inode->i_mapping;
+	struct address_space *mapping = META_MAPPING(sbi);
 	pgoff_t index = 0, end = LONG_MAX;
 	struct pagevec pvec;
 	long nwritten = 0;
@@ -771,7 +771,7 @@
 	wait_on_all_pages_writeback(sbi);
 
 	filemap_fdatawait_range(sbi->node_inode->i_mapping, 0, LONG_MAX);
-	filemap_fdatawait_range(sbi->meta_inode->i_mapping, 0, LONG_MAX);
+	filemap_fdatawait_range(META_MAPPING(sbi), 0, LONG_MAX);
 
 	/* update user_block_counts */
 	sbi->last_valid_block_count = sbi->total_valid_block_count;