linux-user: Remove redundant null check and replace free by g_free

Report from smatch:

linux-user/syscall.c:3632 do_ioctl_dm(220) info:
 redundant null check on big_buf calling free()

'big_buf' was allocated by g_malloc0, therefore free was also
replaced by g_free.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>