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.