riscv: Get rid of MAX_EARLY_MAPPING_SIZE

At early boot stage, we have a whole PGDIR to map the kernel, so there
is no need to restrict the early mapping size to 128MB. Removing this
define also allows us to simplify some compile time logic.

This fixes large kernel mappings with a size greater than 128MB, as it
is the case for syzbot kernels whose size was just ~130MB.

Note that on rv64, for now, we are then limited to PGDIR size for early
mapping as we can't use PGD mappings (see [1]). That should be enough
given the relative small size of syzbot kernels compared to PGDIR_SIZE
which is 1GB.

[1] https://lore.kernel.org/lkml/20200603153608.30056-1-alex@ghiti.fr/



Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>