test/nvmf: Don't let nvmfcleanup() to fail upon finishing the test

If this function fails due to, e.g. modprobe not being able to unload
kernel modules, the errexit will prevent nvmftestfini() from killing
target applications.

Avoid this scenario by ignoring nvmfcleanup()'s es when called from
nvmftestfini().

Change-Id: I3a05261f71d3dcc6ed06f786b4cc1988fd733ffc
Signed-off-by: Michal Berger <michalx.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1377


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Community-CI: Broadcom CI