riscv: Fix comment regarding kernel mapping overlapping with IS_ERR_VALUE

The current comment states that we check if the 64-bit kernel mapping
overlaps with the last 4K of the address space that is reserved to
error values in create_kernel_page_table, which is not the case since it
is done in setup_vm. But anyway, remove the reference to any function
and simply note that in 64-bit kernel, the check should be done as soon
as the kernel mapping base address is known.

Fixes: db6b84a3 ("riscv: Make sure the kernel mapping does not overlap with IS_ERR_VALUE")
Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>