LU-16972 tests: createmany sets xattr

LU-16972 tests: createmany sets xattr

Add -x option to make createmany set an xattr named
user.createmany when open+create the files.
The xattr content is unique to avoid sharing on the
backend filesystem.
This could be useful for testing EA blocks and ea_inode
feature.

Test-Parameters: trivial
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: Ic2effe1f8cc60065dfda649d4ce003d7f10a135c
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51827
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>