tracing/probes: fix error check in parse_btf_field()

In the Linux kernel, the following vulnerability has been resolved: 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.