perf annotate: Rename 'sum' to 'nr_samples' in struct sym_hist

To make it more clear that it is the sum of all the nr_samples fields in the
addr[] entries, i.e.:

  sym_hist->nr_samples = sum(sym_hist->addr[0 ..  symbol__size(sym)]->nr_samples)

Committer notes:

Taeung had renamed it to total_samples, but using nr_samples, as in the
added explanation above, looks clearer and establishes the direct
connection, making clear it is about the _number_ of samples.

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1500500211-16599-1-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>