Fix atime handling.

Previously, the atime-modifying vnops called ZFS_ACCESSTIME_STAMP()
followed by zfs_inode_update() to update the atime.  However, since atimes
are cached in the znode for delayed writing, the zfs_inode_update()
function would effectively ignore the cached atime by reading it from
the SA.

This commit moves the updating of the atime in the inode into
zfs_tstamp_update_setup() which is called by the ZFS_ACCESSTIME_STAMP()
macro and eliminates the call to zfs_inode_update() in the atime-modifying
vnops.

It's possible the same thing could have been done directly in
zfs_inode_update() but I wasn't sure that it was safe in all cases where
it is called.

The effect is that atime handling is as if "strictatime" were selected;
even if the filesystem is mounted with "relatime".

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #1949

module/zfs/zfs_znode.c

@@ -1123,6 +1123,8 @@ zfs_tstamp_update_setup(znode_t *zp, uint_t flag, uint64_t mtime[2],
 
 	if (flag & ATTR_ATIME) {
 		ZFS_TIME_ENCODE(&now, zp->z_atime);
+		ZTOI(zp)->i_atime.tv_sec = zp->z_atime[0];
+		ZTOI(zp)->i_atime.tv_nsec = zp->z_atime[1];
 	}
 
 	if (flag & ATTR_MTIME) {