riscv/mm/fault: Move FAULT_FLAG_WRITE handling in do_page_fault()

Let's handle the translation of EXC_STORE_PAGE_FAULT to FAULT_FLAG_WRITE
once before looking up the VMA. This makes it easier to extract access
error logic in the next patch.

Signed-off-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>