fs: Convert mpage_readpage to mpage_read_folio

mpage_readpage still works in terms of pages, and has not been audited
for correctness with large folios, so include an assertion that the
filesystem is not passing it large folios.  Convert all the filesystems
to call mpage_read_folio() instead of mpage_readpage().

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>