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".

Works around #1917