LU-14645 utils: optimise setstripe

LU-14645 utils: optimise setstripe

skip some excessive checks:
- do not check the file is on lustre fs, the following ioctl does it;
- do not check the stripe-index is valid, done on MDS side;
- do not check the pool exists for a !PFL file (align with a setstripe
  for PFL files);

Signed-off-by: Vitaly Fertman <c17818@cray.com>
Change-Id: Ia21f85c3ab73a970bad8d11e175c0063ab3a307f
Reviewed-on: https://review.whamcloud.com/46152
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>