The underlying copy operation is more efficient if the size is a
constant, which only happens if this function is inlined in the caller.
Otherwise, we end up calling memcpy for each field.
Force inlining for performance reasons for:
- lib_ring_buffer_do_strcpy,
- lib_ring_buffer_do_strcpy_from_user_inatomic,
- lib_ring_buffer_copy_from_user_inatomic.