[S390] Increase default warning stacksize.

Compiling a kernel with allmodconfig or allyesconfig results in tons
of gcc warnings, because the default maximum stacksize from which on
gcc will emit a warning is just 256 bytes.
Increase this to 2048, so these warnings don't distract from the real
warnings that we need to watch at.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/Kconfig

@@ -289,7 +289,7 @@ config WARN_STACK_SIZE
 	int "Maximum frame size considered safe (128-2048)"
 	range 128 2048
 	depends on WARN_STACK
-	default "256"
+	default "2048"
 	help
 	  This allows you to specify the maximum frame size a function may
 	  have without the compiler complaining about it.