ubifs: Convert do_readpage() to take a folio

ubifs: Convert do_readpage() to take a folio

All the callers now have a folio, so pass it in, and convert do_readpage()
to us folios directly.  Includes unifying the exit paths from this
function and using kmap_local instead of plain kmap.  This function
should now work with large folios, but this is not tested.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>