selftests/mm: switch to bash from sh

Running charge_reserved_hugetlb.sh generates errors if sh is set to
dash:

./charge_reserved_hugetlb.sh: 9: [[: not found
./charge_reserved_hugetlb.sh: 19: [[: not found
./charge_reserved_hugetlb.sh: 27: [[: not found
./charge_reserved_hugetlb.sh: 37: [[: not found
./charge_reserved_hugetlb.sh: 45: Syntax error: "(" unexpected

Switch to using /bin/bash instead of /bin/sh. Make the switch for
write_hugetlb_memory.sh as well which is called from
charge_reserved_hugetlb.sh.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Muhammad Usama Anjum <[e-mail]>
Cc: Muhammad Usama Anjum <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: David Laight <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/mm/charge_reserved_hugetlb.sh

···
-#!/bin/sh
+#!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
 # Kselftest framework requirement - SKIP code is 4.

tools/testing/selftests/mm/write_hugetlb_memory.sh

···
-#!/bin/sh
+#!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
 set -e