cpictl: Suppress messages for unwritable sysfs files

Under certain conditions, e.g. in a KVM guest, a write operation
to /sys/firmware/cpi/set can result in an error message to
stderr while the script returns with a zero exit code, indicating
success.

This can cause confusion if cpictl is called by systemd due to
the existence of a writable /sys/firmware/cpi/. Let's silence
cpictl when writing to the /sys/firmware/cpi/set. This is OK, as
nothing can be inferred from failure or success from the perspective
of the invoking Linux instance.

Fixes: https://github.com/ibm-s390-linux/s390-tools/issues/120

Signed-off-by: Viktor Mihajlovski <mihajlov@linux.ibm.com>
Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>

scripts/cpictl

@@ -109,7 +109,7 @@ fail_with()
 
 cpi_commit()
 {
-	echo 1 > "$CPI_SET"
+	echo 1 > "$CPI_SET" 2> /dev/null
 }
 
 do_length_check()