romfs: Convert romfs_read_folio() to use a folio

Remove the conversion back to struct page and use the folio APIs instead
of the page APIs.  It's probably more trouble than it's worth to support
large folios in romfs, so there are still PAGE_SIZE assumptions in
this function.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20240530202110.2653630-13-willy@infradead.org


Signed-off-by: Christian Brauner <brauner@kernel.org>