perf trace: Always print return value for syscalls returning a pid

The syscalls that were consistently observed were set_robust_list and
rseq. This is because perf cannot find their child process.

This change ensures that the return value is always printed.

Before:
     0.256 ( 0.001 ms): set_robust_list(head: 0x7f09c77dba20, len: 24)                        =
     0.259 ( 0.001 ms): rseq(rseq: 0x7f09c77dc0e0, rseq_len: 32, sig: 1392848979)             =
After:
     0.270 ( 0.002 ms): set_robust_list(head: 0x7f0bb14a6a20, len: 24)                        = 0
     0.273 ( 0.002 ms): rseq(rseq: 0x7f0bb14a70e0, rseq_len: 32, sig: 1392848979)             = 0

Committer notes:

As discussed in the thread in the Link: tag below, these two don't
return a pid, but for syscalls returning one, we need to print the
result and if we manage to find the children in 'perf trace' data
structures, then print its name as well.

Fixes: 11c8e39f5133aed9 ("perf trace: Infrastructure to show COMM strings for syscalls returning PIDs")
Reviewed-by: Howard Chu <[email protected]>
Signed-off-by: Anubhav Shelat <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Dapeng Mi <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Michael Petlan <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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