MIPS: Annotate fall-through in kvm/emulate.c

MIPS: Annotate fall-through in kvm/emulate.c

kvm_compute_return_epc contains a switch statement with an intentional
fall-through from a case handling jal (jump and link) instructions to
one handling j (jump) instructions. With -Wimplicit-fallthrough this
triggers a compile error (due to -Werror being enabled for arch/mips).

This can be reproduced using malta_kvm_defconfig.

Fix this by annotating the intentional fall-through.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org