perf lock contention: fix segfault in `lock contention -b/--use-bpf`

When run on a kernel without BTF info, perf crashes:

    libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
    libbpf: failed to find valid kernel BTF

    Program received signal SIGSEGV, Segmentation fault.
    0x00005555556915b7 in btf.type_cnt ()
    (gdb) bt
    #0  0x00005555556915b7 in btf.type_cnt ()
    #1  0x0000555555691fbc in btf_find_by_name_kind ()
    #2  0x00005555556920d0 in btf.find_by_name_kind ()
    #3  0x00005555558a1b7c in init_numa_data (con=0x7fffffffd0a0) at util/bpf_lock_contention.c:125
    #4  0x00005555558a264b in lock_contention_prepare (con=0x7fffffffd0a0) at util/bpf_lock_contention.c:313
    #5  0x0000555555620702 in __cmd_contention (argc=0, argv=0x7fffffffea10) at builtin-lock.c:2084
    #6  0x0000555555622c8d in cmd_lock (argc=0, argv=0x7fffffffea10) at builtin-lock.c:2755
    #7  0x0000555555651451 in run_builtin (p=0x555556104f00 <commands+576>, argc=3, argv=0x7fffffffea10)
        at perf.c:349
    #8  0x00005555556516ed in handle_internal_command (argc=3, argv=0x7fffffffea10) at perf.c:401
    #9  0x000055555565184e in run_argv (argcp=0x7fffffffe7fc, argv=0x7fffffffe7f0) at perf.c:445
    #10 0x0000555555651b9f in main (argc=3, argv=0x7fffffffea10) at perf.c:553

Check if btf loading failed, and don't do anything with it in
init_numa_data(). This leads to the following error message, instead of
just a crash:

    libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
    libbpf: failed to find valid kernel BTF
    libbpf: kernel BTF is missing at '/sys/kernel/btf/vmlinux', was CONFIG_DEBUG_INFO_BTF enabled?
    libbpf: failed to find valid kernel BTF
    libbpf: Error loading vmlinux BTF: -ESRCH
    libbpf: failed to load BPF skeleton 'lock_contention_bpf': -ESRCH
    Failed to load lock-contention BPF skeleton
    lock contention BPF setup failed

Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Chun-Tse Shao <ctshao@google.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>