udf: fix anchor point detection

According to ECMA 167 rev.  3 (see 3/8.4.2.1), Anchor Volume Descriptor
Pointer should be recorded at two or more anchor points located at sectors
256, N, N - 256, where N - is a largest logical sector number at volume
space.

So we should always try to detect N on UDF volume before trying to find
Anchor Volume Descriptor (i.e.  calling to udf_find_anchor()).

That said, all this patch does is updates the s_last_block even if the
udf_vrs() returns positive value.

Originally written and tested by Yuri Per, ported on latest mainline by me.

Signed-off-by: Yuri Per <Yuri.Per@acronis.com>
Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: Max Lyadvinsky <Max.Lyadvinsky@acronis.com>
Cc: Vladimir Simonov <Vladimir.Simonov@acronis.com>
Cc: Andrew Neporada <Andrew.Neporada@acronis.com>
Cc: Kirill Korotaev <dev@openvz.org>
Signed-off-by: Jan Kara <jack@suse.cz>