sparc64: Prevent stack backtrace false positives on trap frames.

When we fully commit to returning back to kernel mode from
a trap, zero out the regs->magic value to prevent false
positives during stack backtraces.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc64/kernel/rtrap.S

@@ -363,6 +363,7 @@ kern_rtt:	rdpr			%canrestore, %g1
 		brz,pn			%g1, kern_rtt_fill
 		 nop
 kern_rtt_restore:
+		stw			%g0, [%sp + PTREGS_OFF + PT_V9_MAGIC]
 		restore
 		retry