f2fs: relocate is_merged_page

Operations in is_merged_page is related to inner bio cache, move it to
data.c.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index ff79054..e41ffc1 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -1880,6 +1880,7 @@
 /*
  * data.c
  */
+bool is_merged_page(struct f2fs_sb_info *, struct page *, enum page_type);
 void f2fs_submit_merged_bio(struct f2fs_sb_info *, enum page_type, int);
 int f2fs_submit_page_bio(struct f2fs_io_info *);
 void f2fs_submit_page_mbio(struct f2fs_io_info *);