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/20240116090455.3407378-1-usama.anjum@collabora.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>