xfs: don't use xlog_in_core_2_t in struct xlog_in_core

xfs: don't use xlog_in_core_2_t in struct xlog_in_core

Most accessed to the on-disk log record header are for the original
xlog_rec_header.  Make that the main structure, and case for the
single remaining place using other union legs.

This prepares for removing xlog_in_core_2_t entirely.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>