xfs: rename struct xfs_legacy_ictimestamp

Rename struct xfs_legacy_ictimestamp to struct xfs_log_legacy_timestamp
as it is a type used for logging timestamps with no relationship to the
in-core inode.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>