selinux: Pass security pointer to determine_inode_label()

Right now selinux_determine_inode_label() works on security pointer of
current task. Soon I need this to work on a security pointer retrieved
from a set of creds. So start passing in a pointer and caller can
decide where to fetch security pointer from.

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>