Fix typos

Correct various typos in the comments and tests.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Andrea Gelmini <andrea.gelmini@gelma.net>
Closes #10423

tests/zfs-tests/cmd/mkbusy/mkbusy.c

@@ -102,7 +102,7 @@ main(int argc, char *argv[])
 
 		/*
 		 * The argument supplied doesn't exist. Copy the path, and
-		 * remove the trailing slash if presnt.
+		 * remove the trailing slash if present.
 		 */
 		if ((arg = strdup(argv[0])) == NULL)
 			fail("strdup", 1);