f2fs: Fix not used variable 'index'

Fix the following compilation warning:
fs/f2fs/data.c:2391:10: warning: variable ‘index’ set but not used
[-Wunused-but-set-variable]
 2391 |  pgoff_t index;

Only define and set the variable index when the CONFIG_F2FS_FS_COMPRESSION
is enabled.

Fixes: db92e6c ("f2fs: convert f2fs_mpage_readpages() to use folio")
Signed-off-by: Zeng Heng <zengheng4@huawei.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: henryZe

fs/f2fs/data.c

@@ -2385,10 +2385,10 @@ static int f2fs_mpage_readpages(struct inode *inode,
 		.nr_cpages = 0,
 	};
 	pgoff_t nc_cluster_idx = NULL_CLUSTER;
+	pgoff_t index;
 #endif
 	unsigned nr_pages = rac ? readahead_count(rac) : 1;
 	unsigned max_nr_pages = nr_pages;
-	pgoff_t index;
 	int ret = 0;
 
 	map.m_pblk = 0;
@@ -2406,9 +2406,9 @@ static int f2fs_mpage_readpages(struct inode *inode,
 			prefetchw(&folio->flags);
 		}
 
+#ifdef CONFIG_F2FS_FS_COMPRESSION
 		index = folio_index(folio);
 
-#ifdef CONFIG_F2FS_FS_COMPRESSION
 		if (!f2fs_compressed_file(inode))
 			goto read_single_page;