selinux: Implement dentry_create_files_as() hook

selinux: Implement dentry_create_files_as() hook

Calculate what would be the label of newly created file and set that
secid in the passed creds.

Context of the task which is actually creating file is retrieved from
set of creds passed in. (old->security).

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Orabug: 25684456

(backport upstream commit a518b0a5b0d7f3397e065acb956bca9635aa892d)

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Acked-by: James Morris <james.l.morris@oracle.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
Conflict fix:
     security/selinux/hooks.c