riscv/mm/fault: Set FAULT_FLAG_INSTRUCTION flag in do_page_fault()

If the page fault "cause" is EXC_INST_PAGE_FAULT, set the
FAULT_FLAG_INSTRUCTION flag to let handle_mm_fault() and friends know
about it. This has no functional changes because RISC-V uses the default
arch_vma_access_permitted() implementation, which always returns true.
However, dax_pmd_fault(), for example, has a tracepoint that uses
FAULT_FLAG_INSTRUCTION, so we might as well set it.

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