perf cgroup: Avoid needless closing of unopened fd

Do not bother with close() if fd is not valid, just to silence valgrind:

    $ valgrind ./perf script
    ==59169== Memcheck, a memory error detector
    ==59169== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
    ==59169== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
    ==59169== Command: ./perf script
    ==59169==
    ==59169== Warning: invalid file descriptor -1 in syscall close()
    ==59169== Warning: invalid file descriptor -1 in syscall close()
    ==59169== Warning: invalid file descriptor -1 in syscall close()
    ==59169== Warning: invalid file descriptor -1 in syscall close()
    ==59169== Warning: invalid file descriptor -1 in syscall close()
    ==59169== Warning: invalid file descriptor -1 in syscall close()
    ==59169== Warning: invalid file descriptor -1 in syscall close()
    ==59169== Warning: invalid file descriptor -1 in syscall close()

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200417132330.119407-1-tommi.t.rantala@nokia.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>