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: db92e6c7 ("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>