SELinux: clear parent death signal on SID transitions

Clear parent death signal on SID transitions to prevent unauthorized
signaling between SIDs.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Eric Paris <eparis@parisplace.org>
Signed-off-by: James Morris <jmorris@localhost.localdomain>
Signed-off-by: Adrian Bunk <bunk@kernel.org>