xfs: use a struct timespec64 for the in-core crtime

struct xfs_icdinode is purely an in-memory data structure, so don't use
a log on-disk structure for it.  This simplifies the code a bit, and
also reduces our include hell slightly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
[darrick: fix a minor indenting problem in xfs_trans_ichgtime]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>