bpf jit: Let the powerpc jit handle negative offsets

Now the helper function from filter.c for negative offsets is exported,
it can be used it in the jit to handle negative offsets.

First modify the asm load helper functions to handle:
- know positive offsets
- know negative offsets
- any offset

then the compiler can be modified to explicitly use these helper
when appropriate.

This fixes the case of a negative X register and allows to lift
the restriction that bpf programs with negative offsets can't
be jited.

Tested-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jan Seiffert <kaffeemonster@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/powerpc/net/bpf_jit.h

@@ -48,7 +48,13 @@
 /*
  * Assembly helpers from arch/powerpc/net/bpf_jit.S:
  */
-extern u8 sk_load_word[], sk_load_half[], sk_load_byte[], sk_load_byte_msh[];
+#define DECLARE_LOAD_FUNC(func)	\
+	extern u8 func[], func##_negative_offset[], func##_positive_offset[]
+
+DECLARE_LOAD_FUNC(sk_load_word);
+DECLARE_LOAD_FUNC(sk_load_half);
+DECLARE_LOAD_FUNC(sk_load_byte);
+DECLARE_LOAD_FUNC(sk_load_byte_msh);
 
 #define FUNCTION_DESCR_SIZE	24