perf evlist: Change the COMM when preparing the workload

It was reported that --exclude-perf wasn't working, as tracepoints were
appearing in 'perf script' output as having the 'perf' COMM, that is
just the window in evlist__prepare_workload() after the fork() and
before the execvp() call for workloads specified in the command line.

Example:

  # perf record -e kmem:kmalloc --filter 'bytes_alloc<650 && bytes_alloc>620' --exclude-perf -e kmem:kfree --exclude-perf -aR sleep 30

Then:

  # perf script
          perf 15905 [009] 1498.356094: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=(nil)
          perf 15905 [009] 1498.356116: kmem:kfree: call_site=free_bprm+0x8f ptr=(nil)
          perf 15905 [009] 1498.356116: kmem:kfree: call_site=do_execveat_common+0x19d ptr=0xffff9cf750421c00
          perf 15905 [009] 1498.356138: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=(nil)
          perf 15905 [009] 1498.356148: kmem:kfree: call_site=free_bprm+0x8f ptr=(nil)
          perf 15905 [009] 1498.356148: kmem:kfree: call_site=do_execveat_common+0x19d ptr=0xffff9cf750421c00
          perf 15905 [009] 1498.356168: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=(nil)
          perf 15905 [009] 1498.356176: kmem:kfree: call_site=free_bprm+0x8f ptr=(nil)
  <SNIP>
          perf 15905 [009] 1498.356348: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=(nil)
          perf 15905 [014] 1498.356386: kmem:kfree: call_site=security_compute_sid.part.0+0x3b2 ptr=(nil)
          perf 15905 [014] 1498.356423: kmem:kfree: call_site=load_elf_binary+0x207 ptr=0xffff9cf5b2a34220
          perf 15905 [014] 1498.356694: kmem:kfree: call_site=__free_slab+0xb5 ptr=0xffff9cf6d0b3b000
         sleep 15905 [014] 1498.356739: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=(nil)

Use prctl() to show that that is just the preparation of the workload:

  # perf script
     perf-exec 19036 [009] 2199.357582: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=(nil)
     perf-exec 19036 [009] 2199.357604: kmem:kfree: call_site=free_bprm+0x8f ptr=(nil)
     perf-exec 19036 [009] 2199.357604: kmem:kfree: call_site=do_execveat_common+0x19d ptr=0xffff9cf786459800
     perf-exec 19036 [009] 2199.357630: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=(nil)
  <SNIP>
     perf-exec 19036 [000] 2199.358277: kmem:kfree: call_site=__free_slab+0xb5 ptr=0xffff9cf786fb9c00
     perf-exec 19036 [000] 2199.358278: kmem:kfree: call_site=__free_slab+0xb5 ptr=0xffff9cf786458200
     perf-exec 19036 [000] 2199.358279: kmem:kfree: call_site=__free_slab+0xb5 ptr=0xffff9cf786458600
         sleep 19036 [000] 2199.358316: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=(nil)
         sleep 19036 [000] 2199.358323: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=(nil)
         sleep 19036 [000] 2199.358330: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=0xffff9cf58be2d000
         sleep 19036 [000] 2199.358337: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=0xffff9cf58be2d000
         sleep 19036 [000] 2199.358339: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=0xffff9cf58be2d000
         sleep 19036 [000] 2199.358341: kmem:kfree: call_site=perf_event_mmap+0x279 ptr=0xffff9cf58be2d000

Reporter: zhanweiw <wingfancy@hotmail.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=212213
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>