perf hists: Trim libtraceevent trace_seq buffers

When we use libtraceevent to format trace event fields into printable
strings to use in hist entries it is important to trim it from the
default 4 KiB it starts with to what is really used, to reduce the
memory footprint, so use realloc(seq.buffer, seq.len + 1) when returning
the seq.buffer formatted with the fields contents.

Reported-and-Tested-by: Wang Nan <wangnan0@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-t3hl7uxmilrkigzmc90rlhk2@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>