afs: convert afs_writepages_region() to use filemap_get_folios_tag()

afs: convert afs_writepages_region() to use filemap_get_folios_tag()

Convert to use folios throughout.  This function is in preparation to
remove find_get_pages_range_tag().

Also modify this function to write the whole batch one at a time, rather
than calling for a new set every single write.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Vishal Moola (Oracle) <[email protected]>
Tested-by: David Howells <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>