f2fs: split drop_inmem_pages from commit_inmem_pages

Split drop_inmem_pages from commit_inmem_pages for code readability,
and prepare for the following modification.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index 9445a34..fa4bbe0 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -494,7 +494,7 @@
 
 			/* some remained atomic pages should discarded */
 			if (f2fs_is_atomic_file(inode))
-				commit_inmem_pages(inode, true);
+				drop_inmem_pages(inode);
 
 			/* should remain fi->extent_tree for writepage */
 			f2fs_destroy_extent_node(inode);