fixrange_init operates at PMD-granularity and expects the addresses to
be PMD-size aligned, but currently that might not be the case for
PKMAP_BASE unless it is defined properly, so ensure a correct alignment
is used before passing the address to fixrange_init.
fixed mappings: only align the start address that is passed to
fixrange_init rather than the value before adding the size, as we may
end up with uninitialised upper part of the range.