ecryptfs: Use struct_size to improve process_response + send_miscdev

ecryptfs: Use struct_size to improve process_response + send_miscdev

Use struct_size(), which provides additional compile-time checks for
structures with flexible array members (e.g., __must_be_array()), to
determine the allocation size for a new 'struct ecryptfs_message'.

In send_miscdev(), reuse 'msg_size' instead of recalculating it.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Tyler Hicks <code@tyhicks.com>