perf top: Lower message level for failure on synthesizing events for pre-existing...

perf top: Lower message level for failure on synthesizing events for pre-existing BPF programs

Move it from being a pr_warning() to a pr_debug(). Also capitalize BPF
and explain what gets missing when we're not able to synthesize these
events: we'll not be able to resolve symbols, etc.

Reported-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Yonghong Song <yhs@fb.com>
Link: https://lkml.kernel.org/n/tip-whpnfnw6xtd939odgt9bw9as@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>