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

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 <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Namhyung Kim <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>