perf test: Handle fd gaps in test__dso_data_reopen

https://github.com/beaker-project/restraint/issues/215

 describes a file
descriptor leak which revealed the test failure described here.

The 'DSO data reopen' perf test assumes that RLIMIT_NOFILE limits the
number of open file descriptors, but it actually limits newly opened
file descriptors. When the file descriptor limit is reduced, file
descriptors already open remain open regardless of the new limit. This
test failure does not occur if open file descriptors are contiguous,
beginning at zero.

The following command triggers this perf test failure.

perf test 'DSO data reopen' 3>/dev/null 8>/dev/null

This patch determines the file descriptor limit by opening four files
and then closing them. The limit is set to the fourth file descriptor,
leaving only the first three available because any newly opened file
descriptor must be less than the limit.

Signed-off-by: Eirik Fuller <efuller@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Michael Petlan <mpetlan@redhat.com>
LPU-Reference: 20210626023825.1398547-1-efuller@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>