Call gethrtime() only once per new txg creation

When transitioning current open TXG into QUIESCE state and opening
a new one txg_quiesce() calls gethrtime():

• to mark the birth time of the new TXG
• to record the SPA txg history kstat
• implicitely inside spa_txg_history_add()

These timestamps are practically the same, so that the first one
can be used instead of the other two. The only visible difference
is that inside spa_txg_history_add() the time spent in kmem_zalloc()
will be counted towards the opened TXG.

Since at this point the new TXG already exists (tx->tx_open_txg
has been already incremented) it is actually a correct accounting.

In any case this extra work is only happening when spa_txg_history
kstat is activated (i.e. zfs_txg_history > 0) and doesn't affect
the normal processing in any way.

Signed-off-by: Cyril Plisko <cyril.plisko@mountall.com>
Issue #2075