MIPS: math-emu: Fix delay-slot emulation cache incoherency

Correct a cache coherency regression introduced with be1664c4 [Another
round of fixes for the fp emulator.] for the emulation frame used in
delay-slot emulation.

Two instructions are copied into the frame and as from the commit
referred a cache synchronisation call is made for the second instruction
aka `badinst' of the two only.  The `flush_cache_sigtramp' interface is
reused that guarantees that synchronisation will be made for 8 bytes or
2 instructions starting from the address requested, although if cache
lines are wider then a larger area may be synchronised.

Change the call to point to the first of the two instructions aka `emul'
instead, removing unpredictable behaviour resulting from cache
incoherency.

This bug only ever manifested itself on systems implementing 4-byte
cache lines, typically MIPS I systems, causing all kinds of weirdness.
This is because the sequence of two instructions starting from `emul' is
8-byte aligned and for 8-byte or wider cache lines the line synchronised
will span both, so the vast majority of systems have escaped unharmed.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9698/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/math-emu/dsemul.c

@@ -94,7 +94,7 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
 	regs->cp0_epc = ((unsigned long) &fr->emul) |
 		get_isa16_mode(regs->cp0_epc);
 
-	flush_cache_sigtramp((unsigned long)&fr->badinst);
+	flush_cache_sigtramp((unsigned long)&fr->emul);
 
 	return SIGILL;		/* force out of emulation loop */
 }