bpf: Fix a warning message in mark_ptr_not_null_reg()

The WARN_ON() argument is a condition, not an error message.  So this
code will print a stack trace but will not print the warning message.
Fix that and also change it to only WARN_ONCE().

Fixes: 4ddb741 ("bpf: Extract nullable reg type conversion into a helper function")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/YCzJlV3hnF%2Ft1Pk4@mwanda

kernel/bpf/verifier.c

@@ -1120,7 +1120,7 @@ static void mark_ptr_not_null_reg(struct bpf_reg_state *reg)
 		reg->type = PTR_TO_RDWR_BUF;
 		break;
 	default:
-		WARN_ON("unknown nullable register type");
+		WARN_ONCE(1, "unknown nullable register type");
 	}
 }