dmu_tx kstat cleanup

A few counters in the dmu_tx kstats are obsolete or no longer
bumped properly.

• The sync task restructuring commit 13fe019870c8779bf2f5b3ff731b512cf89133ef removed the code that bumpted dmu_tx_quota. The counter is now bumped in two cases, instead of just the one case as before (after the result of dsl_dataset_check_quota call). The second case is where we check the requested reservation against the actual pool size, as this is an implicit quota of sorts.

• The write throttle restructuring commit e8b96c6007bf97cdf34869c1ffbd0ce753873a3d makes dmu_tx_how and dmu_tx_inflight obsolete, so they are removed.

Signed-off-by: Kohsuke Kawaguchi <kk@kohsuke.org>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1914