MIPS: clear execution hazard after changing FTLB enable

On current P-series cores from Imagination the FTLB can be enabled or
disabled via a bit in the Config6 register, and an execution hazard is
created by changing the value of bit. The ftlb_disable function already
cleared that hazard but that does no good for other callers. Clear the
hazard in the set_ftlb_enable function that creates it, and only for the
cores where it applies.

This has the effect of reverting c982c6d6c48b ("MIPS: cpu-probe: Remove
cp0 hazard barrier when enabling the FTLB") which was incorrect.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Fixes: c982c6d6c48b ("MIPS: cpu-probe: Remove cp0 hazard barrier when enabling the FTLB")
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14023/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/cpu-probe.c

@@ -376,8 +376,6 @@ static int __init ftlb_disable(char *s)
 		return 1;
 	}
 
-	back_to_back_c0_hazard();
-
 	config4 = read_c0_config4();
 
 	/* Check that FTLB has been disabled */
@@ -560,6 +558,7 @@ static int set_ftlb_enable(struct cpuinfo_mips *c, enum ftlb_flags flags)
 		}
 
 		write_c0_config6(config);
+		back_to_back_c0_hazard();
 		break;
 	case CPU_I6400:
 		/* There's no way to disable the FTLB */