ceph: use struct_size() helper in __ceph_pool_perm_get()

Use struct_size() to calculate the number of bytes to be allocated.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>