LU-12469 mdd: handle migrate case with SELinux

LU-12469 mdd: handle migrate case with SELinux

In case a metadata object is created for migration purpose,
its security context should not be initialized. The
security.selinux xattr will be copied after creation, just like
any other xattr, so that the migrated object has the right security
context.

Lustre-change: https://review.whamcloud.com/36684
Lustre-commit: 8a60fa2e2fcd28c2772d90e76d36430d30b01905

Test-Parameters: clientselinux mdtcount=4 envdefinitions=ONLY=230 testlist=sanity,sanity,sanity,sanity
Test-Parameters: clientselinux mdtcount=4 testlist=sanity,recovery-small,sanity-sec
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I0bc274426c003f8081da2f4d1e8e6c12a70b9930
Reviewed-on: https://review.whamcloud.com/37031
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>