mm: convert page_add_file_rmap() to use a folio internally

The API for page_add_file_rmap() needs to be page-based, because we can
add mappings of individual pages.  But inside the function, we want to
only call compound_head() once and then use the folio APIs instead of the
page APIs that each call compound_head().

Link: https://lkml.kernel.org/r/20230111142915.1001531-8-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>