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: db92e6c729d8 ("f2fs: convert f2fs_mpage_readpages() to use folio")
Signed-off-by: Zeng Heng <zengheng4@xxxxxxxxxx>