ZTS: Fix ksh path in btree tests

Every other ksh script has /bin/ksh in the shebang.
If we're going to assume a path, we can at least be consistent.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9845

tests/zfs-tests/tests/functional/btree/btree_negative.ksh

@@ -1,4 +1,4 @@
-#!/usr/bin/ksh -p
+#!/bin/ksh -p
 
 #
 # This file and its contents are supplied under the terms of the

tests/zfs-tests/tests/functional/btree/btree_positive.ksh

@@ -1,4 +1,4 @@
-#!/usr/bin/ksh -p
+#!/bin/ksh -p
 
 #
 # This file and its contents are supplied under the terms of the