Message padding allocates buffer space unnecessarily

[AaronFeickert]

The recent PR 4753 changes message encryption to use an in-place BytesMut construction to avoid unnecessary allocation. When applying padding prior to encryption, it reserves extra space in the buffer to accommodate the padding before adding it. However, it reserves too much space.

The documentation for BytesMut indicates that reserve should be provided the number of additional bytes needed in the buffer, not the total number of bytes needed. As a result, the current code appears to allocate the buffer to accommodate twice the message length plus padding, which appears to negate the desired benefit of using the BytesMut approach.

Changing the linked line to message.reserve(padding_length); seems to be sufficient to avoid unnecessary reallocation, while accounting for the extra space needed for padding.