selinux: change security_compute_sid to return the ssid or tsid on match

If the end result of a security_compute_sid() computation matches the
ssid or tsid, return that SID rather than looking it up again. This
avoids the problem of multiple initial SIDs that map to the same
context.

Cc: stable@vger.kernel.org
Reported-by: Guido Trentalancia <guido@trentalancia.com>
Fixes: ae254858 ("selinux: introduce an initial SID for early boot processes")
Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Tested-by: Guido Trentalancia <guido@trentalancia.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>