powerpc/ftrace: Fix inverted check of create_branch()

In commit 24a1bdc3, "Fix ABIv2 issues with __ftrace_make_call", Anton
changed the logic that creates and patches the branch, and added a
thinko in the check of create_branch(). create_branch() returns the
instruction that was generated, so if we get zero then it succeeded.

The result is we can't ftrace modules:

  Branch out of range
  WARNING: at ../kernel/trace/ftrace.c:1638
  ftrace failed to modify [<d000000004ba001c>] fuse_req_init_context+0x1c/0x90 [fuse]

We should probably fix patch_instruction() to do that check and make the
API saner, but that's a separate patch. For now just invert the test.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/ftrace.c

@@ -320,7 +320,7 @@ __ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
 	}
 
 	/* Ensure branch is within 24 bits */
-	if (create_branch(ip, rec->arch.mod->arch.tramp, BRANCH_SET_LINK)) {
+	if (!create_branch(ip, rec->arch.mod->arch.tramp, BRANCH_SET_LINK)) {
 		printk(KERN_ERR "Branch out of range");
 		return -EINVAL;
 	}