Improve reporting of tx assignment wait times

Some callers of dmu_tx_assign() use the TXG_NOWAIT flag and call
dmu_tx_wait() themselves before retrying if the assignment fails.
The wait times for such callers are not accounted for in the
dmu_tx_assign kstat histogram, because the histogram only records
time spent in dmu_tx_assign().  This change moves the histogram
update to dmu_tx_wait() to properly account for all time spent there.

One downside of this approach is that it is possible to call
dmu_tx_wait() multiple times before successfully assigning a
transaction, in which case the cumulative wait time would not be
recorded.  However, this case should not often arise in practice,
because most callers currently use one of these forms:

  dmu_tx_assign(tx, TXG_WAIT);
  dmu_tx_assign(tx, waited ?  TXG_WAITED : TXG_NOWAIT);

The first form should make just one call to dmu_tx_delay() inside of
dmu_tx_assign(). The second form retries with TXG_WAITED if the first
assignment fails and incurs a delay, in which case no further waiting
is performed.  Therefore transaction delays normally occur in one
call to dmu_tx_wait() so the histogram should be fairly accurate.

Another possible downside of this approach is that the histogram will
no longer record overhead outside of dmu_tx_wait() such as in
dmu_tx_try_assign(). While I'm not aware of any reason for concern on
this point, it is conceivable that lock contention, long list
traversal, etc. could cause assignment delays that would not be
reflected in the histogram.  Therefore the histogram should strictly
be used for visibility in to the normal delay mechanisms and not as a
profiling tool for code performance.

Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes #1915