btrfs: convert io_ctl_prepare_pages() to work on folios

Retrieve folios instead of pages and work on them throughout.  Removes
a few calls to compound_head() and a reference to page->mapping.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: David Sterba <dsterba@suse.com>