mm/thp: make ARCH_ENABLE_SPLIT_PMD_PTLOCK dependent on PGTABLE_LEVELS > 2

ARCH_ENABLE_SPLIT_PMD_PTLOCK is irrelevant unless there are more than two
page table levels including PMD (also per
Documentation/vm/split_page_table_lock.rst).  Make this dependency
explicit on remaining platforms i.e x86 and s390 where
ARCH_ENABLE_SPLIT_PMD_PTLOCK is subscribed.

Link: https://lkml.kernel.org/r/1622013501-20409-1-git-send-email-anshuman.khandual@arm.com
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: Gerald Schaefer <gerald.schaefer@linux.ibm.com> # s390
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>