f2fs: Use a folio in f2fs_wait_on_block_writeback()

Fetch a folio from the pagecache and use it.  Removes two calls
to compound_head().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>