bpf: rename stack trace map operations

In the following patches queue and stack maps (FIFO and LIFO
datastructures) will be implemented.  In order to avoid confusion and
a possible name clash rename stack_map_ops to stack_trace_map_ops

Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@polito.it>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>