tracing/probes: fix error check in parse_btf_field()

btf_find_struct_member() might return NULL or an error via the
ERR_PTR() macro. However, its caller in parse_btf_field() only checks
for the NULL condition. Fix this by using IS_ERR() and returning the
error up the stack.

Link: https://lore.kernel.org/all/20240527094351.15687-1-clopez@suse.de/

Fixes: c440adfb ("tracing/probes: Support BTF based data structure field access")
Signed-off-by: Carlos López <clopez@suse.de>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

kernel/trace/trace_probe.c

@@ -554,6 +554,10 @@ static int parse_btf_field(char *fieldname, const struct btf_type *type,
 			anon_offs = 0;
 			field = btf_find_struct_member(ctx->btf, type, fieldname,
 						       &anon_offs);
+			if (IS_ERR(field)) {
+				trace_probe_log_err(ctx->offset, BAD_BTF_TID);
+				return PTR_ERR(field);
+			}
 			if (!field) {
 				trace_probe_log_err(ctx->offset, NO_BTF_FIELD);
 				return -ENOENT;