MIPS: Prevent "restoration" of MSA context in non-MSA kernels

If a kernel doesn't support MSA context (ie. CONFIG_CPU_HAS_MSA=n) then
it will only keep 64 bits per FP register in thread context, and the
calls to set_fpr64 in restore_msa_extcontext will overrun the end of the
FP register context into the FCSR & MSACSR values. GCC 6.x has become
smart enough to detect this & complain like so:

    arch/mips/kernel/signal.c: In function 'protected_restore_fp_context':
    ./arch/mips/include/asm/processor.h:114:17: error: array subscript is above array bounds [-Werror=array-bounds]
      fpr->val##width[FPR_IDX(width, idx)] = val;   \
      ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
    ./arch/mips/include/asm/processor.h:118:1: note: in expansion of macro 'BUILD_FPR_ACCESS'
     BUILD_FPR_ACCESS(64)

The only way to trigger this code to run would be for a program to set
up an artificial extended MSA context structure following a sigframe &
execute sigreturn. Whilst this doesn't allow a program to write to any
state that it couldn't already, it makes little sense to allow this
"restoration" of MSA context in a system that doesn't support MSA.

Fix this by killing a program with SIGSYS if it tries something as crazy
as "restoring" fake MSA context in this way, also fixing the build error
& allowing for most of restore_msa_extcontext to be optimised out of
kernels without support for MSA.
Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reported-by: Michal Toman <michal.toman@imgtec.com>
Fixes: bf82cb30 ("MIPS: Save MSA extended context around signals")
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Michal Toman <michal.toman@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: stable <stable@vger.kernel.org> # v4.3+
Patchwork: https://patchwork.linux-mips.org/patch/13164/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/signal.c

@@ -195,6 +195,9 @@ static int restore_msa_extcontext(void __user *buf, unsigned int size)
 	unsigned int csr;
 	int i, err;
 
+	if (!config_enabled(CONFIG_CPU_HAS_MSA))
+		return SIGSYS;
+
 	if (size != sizeof(*msa))
 		return -EINVAL;
 
@@ -398,8 +401,8 @@ int protected_restore_fp_context(void __user *sc)
 	}
 
 fp_done:
-	if (used & USED_EXTCONTEXT)
-		err |= restore_extcontext(sc_to_extcontext(sc));
+	if (!err && (used & USED_EXTCONTEXT))
+		err = restore_extcontext(sc_to_extcontext(sc));
 
 	return err ?: sig;
 }