f2fs: Convert gc_data_segment() to use a folio

Use f2fs_get_read_data_folio() instead of f2fs_get_read_data_page().
Saves a hidden call to compound_head() in f2fs_put_page().

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