LU-9484 llite: eat -EEXIST on setting trunsted.lov

Tools like rsync, tar, cp may copy and restore the xattrs on a file.
The client previously ignored the setting of trusted.lov/lustre.lov
if the layout had already been specified, to avoid causing these
tools to fail for no reason.

For PFL files we still need to silently eat -EEXIST on setting these
attributes to avoid problems.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: Ic6d359c0f3557d4a21e950a53f57e38ae97a40da
Reviewed-on: https://review.whamcloud.com/27126
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>