selinux: Create a common helper to determine an inode label [ver #3]

Create a common helper function to determine the label for a new inode.
This is then used by:

	- may_create()
	- selinux_dentry_init_security()
	- selinux_inode_init_security()

This will change the behaviour of the functions slightly, bringing them
all into line.

Suggested-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <pmoore@redhat.com>