perf report: Introduce -n/--show-nr-samples

[acme@doppio pahole]$ perf report -ns comm,dso,symbol -d /lib64/libc-2.10.1.so -C pahole | head -17
    21.94%      32101  [.] _int_malloc
    20.10%      29402  [.] __GI_strcmp
    16.77%      24533  [.] __tsearch
    12.61%      18450  [.] malloc_consolidate
     6.42%       9394  [.] _int_free
     6.28%       9191  [.] __tfind
     4.56%       6678  [.] __GI___libc_free
     4.46%       6520  [.] _IO_vfprintf_internal
     2.59%       3786  [.] __malloc
     1.17%       1716  [.] __GI_memcpy
[acme@doppio pahole]$

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
LKML-Reference: <1247325517-12272-5-git-send-email-acme@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>