amcheck: Use correct varlena size accessor in bt_normalize_tuple()

amcheck: Use correct varlena size accessor in bt_normalize_tuple()

bt_normalize_tuple() uses VARSIZE() to get the size of varlena, even though
it's not yet known, that it has a 4-byte header.  Fix this by replacing a
accessor with a universal VARSIZE_ANY().

Backpatch to all supported versions.

Reported-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/7ckc7oka4bvafkf5bwlqs6ygrhlsbhz25ppozfch7zbuxcx3rf%40e4pr4oqenalc
Author: Andrey Borodin <x4mmm@yandex-team.ru>
Reviewed-by: Alexander Korotkov <aekorotkov@gmail.com>
Backpatch-through: 14