f2fs: Add f2fs_find_data_folio()

Convert f2fs_find_data_page() to f2fs_find_data_folio() and add a
compatibility wrapper.  Saves six hidden calls to compound_head().
This was the last caller of f2fs_get_read_data_page(), so remove
the compatibility wrapper.

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