LU-9351 pfl: setstripe to existing file

LU-9351 pfl: setstripe to existing file

It's ok to setstripe to an existing file when the file has no
LOVEA created yet, so we should not use exlusive open for the
'lfs setstripe'.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I2090ba2940b391f7853cc06ccdb9cd842ad6f984
Reviewed-on: https://review.whamcloud.com/26655
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>