Fix various typos

Correct an assortment of typos throughout the code base.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Closes #11774

cmd/ztest/ztest.c

@@ -5979,7 +5979,7 @@ ztest_fault_inject(ztest_ds_t *zd, uint64_t id)
 		    vd0->vdev_resilver_txg != 0)) {
 			/*
 			 * Make vd0 explicitly claim to be unreadable,
-			 * or unwriteable, or reach behind its back
+			 * or unwritable, or reach behind its back
 			 * and close the underlying fd.  We can do this if
 			 * maxfaults == 0 because we'll fail and reexecute,
 			 * and we can do it if maxfaults >= 2 because we'll