mm: mmap: Align unhinted maps to highest anon folio order

When allocating large anonymous folios, we want to maximize our chances
of being able to use the highest order we support. Since one of the
constraints is that a folio has to be mapped naturally aligned, let's
have mmap default to that alignment when user space does not provide a
hint.

With this in place, an extra 2% of all allocated anonymous memory
belongs to a folio of the highest order, when compiling the kernel.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>