linux-user/syscall: Replace g_malloc0 + memcpy with g_memdup

linux-user/syscall: Replace g_malloc0 + memcpy with g_memdup

No need to use g_malloc0 to zero the memory if we memcpy to
the whole buffer afterwards anyway. Actually, there is even
a function which combines both steps, g_memdup, so let's use
this function here instead.

Signed-off-by: Thomas Huth <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>