LU-11883 nodemap: make deny_unknown visible on default nodemap

LU-11883 nodemap: make deny_unknown visible on default nodemap

deny_unknown can be set on the 'default' nodemap, but its value
cannot be read, neither with 'lctl get_param' nor by reading the
file /proc/fs/lustre/nodemap/default/deny_unknown directly.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: Idc8db01a8d32f0ae071f92307843379f4c02571c
Reviewed-on: https://review.whamcloud.com/34090
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>