[SPARC64]: Don't clobber alt-global %g4 on window fixups.

If we are returning back to kernel mode, %g4 could be live
(for example, in the case where we window spill in the etrap
code).  So do not change it's value if going back to kernel.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc64/kernel/winfixup.S

@@ -160,7 +160,7 @@ spill_fixup:
 	andcc		%g1, TSTATE_PRIV, %g0
 	saved
 	and		%g1, TSTATE_CWP, %g1
-	be,pn		%xcc, window_scheisse_from_user_common
+	be,a,pn		%xcc, window_scheisse_from_user_common
 	 mov		FAULT_CODE_WRITE | FAULT_CODE_DTLB | FAULT_CODE_WINFIXUP, %g4
 	retry