KVM: MIPS/MMU: Handle dirty logging on GPA faults

Update kvm_mips_map_page() to handle logging of dirty guest physical
pages. Upcoming patches will propagate the dirty bit to the GVA page
tables.

A fast path is added for handling protection bits that can be resolved
without calling into KVM, currently just dirtying of clean pages being
written to.

The slow path marks the GPA page table entry writable only on writes,
and at the same time marks the page dirty in the dirty page logging
bitmask.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org