It should be in a proper mnt namespace when accessing the file.

I think this had no problem since the build-id was actually read from
map__load() -> dso__load() already.  But I'd like to change it in the
following commit.
Acked-by: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20201012070214.2074921-4-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>