mm: use ARCH_PKEY_BITS to define VM_PKEY_BITN

Use the new CONFIG_ARCH_PKEY_BITS to simplify setting these bits
for different architectures.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20240822151113.1479789-4-joey.gouly@arm.com


Signed-off-by: Will Deacon <will@kernel.org>