nvme/rc: silence fcloop cleanup failures

nvme/rc: silence fcloop cleanup failures

When the ctl file is missing we are logging

  tests/nvme/rc: line 265: /sys/class/fcloop/ctl/del_target_port: No such file or directory
  tests/nvme/rc: line 257: /sys/class/fcloop/ctl/del_local_port: No such file or directory
  tests/nvme/rc: line 249: /sys/class/fcloop/ctl/del_remote_port: No such file or directory

because the first redirect operator fails. Also it's not possible to
redirect the 'echo' error to /dev/null, because it's a builtin command
which escapes the stderr redirect operator (why?).

Anyway, the simplest way to catch this error is to first check if the
control file exists before attempting to write to it.

Signed-off-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>