arm64: Support ARM64_VA_BITS=52 when setting ARCH_MMAP_RND_BITS_MAX

When the 52-bit virtual addressing was introduced the select like
ARCH_MMAP_RND_BITS_MAX logic was never updated to account for it.
Because of that the rnd max bits knob is set to the default value of 18
when ARM64_VA_BITS=52.
Fix this by setting ARCH_MMAP_RND_BITS_MAX to the same value that would
be used if 48-bit addressing was used. Higher values can't used here
because 52-bit addressing is used only if the caller provides a hint to
mmap, with a fallback to 48-bit. The knob in question is an upper bound
for what the user can set in /proc/sys/vm/mmap_rnd_bits, which in turn
is used to determine how many random bits can be inserted into the base
address used for mmap allocations. Since 48-bit allocations are legal
with ARM64_VA_BITS=52, we need to make sure that the base address is
small enough to facilitate this.

Fixes: b6d00d47 ("arm64: mm: Introduce 52-bit Kernel VAs")
Signed-off-by: Kornel Dulęba <korneld@google.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lore.kernel.org/r/20250417114754.3238273-1-korneld@google.com


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