Blackfin arch: Fix random crash issue found by Michael.

This is fixes a problem where we could jump to the wrong address. By
doing a "p0 = reti; jump (p0)". If a different, higher level interrupt
came in, just before, rather than returning to the calling function, we
would return to a random place in the kernel.

This very elegant fix from Bernd grabs the return location off the
stack, and places it into P0, so when we do a return, it goes to the
correct place.
Signed-off-by: NRobin Getz <robin.getz@analog.com>
Signed-off-by: NBryan Wu <bryan.wu@analog.com>

arch/blackfin/kernel/entry.S

@@ -55,8 +55,10 @@ ENTRY(_ret_from_fork)
 	/* do a 'fake' RTI by jumping to [RETI]
 	 * to avoid clearing supervisor mode in child
 	 */
+	r0 = [sp + PT_PC];
+	[sp + PT_P0] = r0;
+
 	RESTORE_ALL_SYS
-	p0 = reti;
 	jump (p0);
 ENDPROC(_ret_from_fork)