LU-13498 sec: fix credentials with nodemap and SSK

LU-13498 sec: fix credentials with nodemap and SSK

When SSK is enabled, credentials are evaluated in new_init_ucred().
In case a nodemap entry is defined with squash UID/GID, it must
prevail over normally mapped UID/GID.

Lustre-change: https://review.whamcloud.com/40140
Lustre-commit: 2bf6442d7d9bd452153e6b1ea08ddaae3dfb3716

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I1adfd98759e5b98ec78f0477846e1820fed5d8b3
Reviewed-on: https://review.whamcloud.com/40960
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>