f2fs: Use a folio more in f2fs_submit_page_bio()

Cache the result of page_folio(fio->page) in a local variable so
we don't have to keep calling it.  Saves a couple of calls to
compound_head() and removes an access to page->mapping.

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