udf: Unify .read_folio for normal and in-ICB files

Switching address_space_operations while a file is used is difficult to
do in a race-free way. To be able to use single address_space_operations
in UDF, make udf_read_folio() handle both normal and in-ICB files.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>