zdtm: fix compilation in test_msg()

cc -g -O2 -Wall -Werror -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0   -c -o msg.o msg.c
msg.c: In function ‘test_msg’:
msg.c:40:6: error: variable ‘len’ set but not used [-Werror=unused-but-set-variable]

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>