arm64: Disable unhandled signal log messages by default

aarch64 unhandled signal kernel messages are very verbose, suggesting
them to be more of a debugging aid:

sigsegv[33]: unhandled level 2 translation fault (11) at 0x00000000, esr
0x92000046, in sigsegv[400000+71000]
CPU: 1 PID: 33 Comm: sigsegv Tainted: G        W        4.15.0-rc3+ #3
Hardware name: linux,dummy-virt (DT)
pstate: 60000000 (nZCv daif -PAN -UAO)
pc : 0x4003f4
lr : 0x4006bc
sp : 0000fffffe94a060
x29: 0000fffffe94a070 x28: 0000000000000000
x27: 0000000000000000 x26: 0000000000000000
x25: 0000000000000000 x24: 00000000004001b0
x23: 0000000000486ac8 x22: 00000000004001c8
x21: 0000000000000000 x20: 0000000000400be8
x19: 0000000000400b30 x18: 0000000000484728
x17: 000000000865ffc8 x16: 000000000000270f
x15: 00000000000000b0 x14: 0000000000000002
x13: 0000000000000001 x12: 0000000000000000
x11: 0000000000000000 x10: 0008000020008008
x9 : 000000000000000f x8 : ffffffffffffffff
x7 : 0004000000000000 x6 : ffffffffffffffff
x5 : 0000000000000000 x4 : 0000000000000000
x3 : 00000000004003e4 x2 : 0000fffffe94a1e8
x1 : 000000000000000a x0 : 0000000000000000

Disable them by default, so they can be enabled using
/proc/sys/debug/exception-trace.

Cc: <stable@vger.kernel.org>
Signed-off-by: Michael Weiser <michael.weiser@gmx.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/traps.c

@@ -57,7 +57,7 @@ static const char *handler[]= {
 	"Error"
 };
 
-int show_unhandled_signals = 1;
+int show_unhandled_signals = 0;
 
 static void dump_backtrace_entry(unsigned long where)
 {