LU-9868 lustre: switch to use of ->d_init()

LU-9868 lustre: switch to use of ->d_init()

Starting with 4.7 kernels the initialization of dentries
is now managed by the VFS layer at allocation time. Any
time a dentry is created by the VFS ll_d_init will be
called.

Linux-commit: 7126bc2e8d60c2a00539bf96b1005f3015be87a5

Change-Id: I02f9b83afd5007658ce88c1010c669d642665d39
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/44135
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>