f2fs: Pass a folio to f2fs_init_read_extent_tree()

The only caller alredy has a folio so pass it in.  Remove 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>