MIPS: bpf: Return error code if the offset is a negative number

Previously, the negative offset was not checked leading to failures
due to trying to load data beyond the skb struct boundaries. Until we
have proper asm helpers in place, it's best if we return ENOSUPP if K
is negative when trying to JIT the filter or 0 during runtime if we
do an indirect load where the value of X is unknown during build time.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Daniel Borkmann <dborkman@redhat.com>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: netdev@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/7123/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>