kernfs: define kernfs_node_dentry

Add a new kernfs api is added to lookup the dentry for a particular
kernfs path.

Signed-off-by: Aditya Kali <adityakali@google.com>
Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
Changelog:
	20151116 - Don't allow user namespaces to bind new subsystems
	20151118 - postpone the FS_USERNS_MOUNT flag until the
	           last patch, until we can convince ourselves it
		   is safe.
	20151207 - Switch to walking up the kernfs path from kn root.
	20151208 - Split out the kernfs change
		 - Style changes
		 - Switch from pr_crit to WARN_ON
		 - Reorder arguments to kernfs_obtain_root
		 - rename kernfs_obtain_root to kernfs_node_dentry
2 files changed