udf: Fix regression in UDF anchor block detection

In some cases it could happen that some block passed test in
udf_check_anchor_block() even though udf_read_tagged() refused to read it later
(e.g. because checksum was not correct).  This patch makes
udf_check_anchor_block() use udf_read_tagged() so that the checking is
stricter.

This fixes the regression (certain disks unmountable) caused by commit
423cf6dc

.

Signed-off-by: Tomas Janousek <tomi@nomi.cz>
Signed-off-by: Jan Kara <jack@suse.cz>