f2fs: drop usage of folio_index

folio_index is only needed for mixed usage of page cache and swap
cache, for pure page cache usage, the caller can just use
folio->index instead.

It can't be a swap cache folio here.  Swap mapping may only call into fs
through `swap_rw` but f2fs does not use that method for swap.

Signed-off-by: Kairui Song <kasong@tencent.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org> (maintainer:F2FS FILE SYSTEM)
Cc: Chao Yu <chao@kernel.org> (maintainer:F2FS FILE SYSTEM)
Cc: linux-f2fs-devel@lists.sourceforge.net (open list:F2FS FILE SYSTEM)
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>