Fix writing of the final block of encrypted, compressed kernel dumps.

Previously any residual data in the final block of a compressed kernel
dump would be written unencrypted. Note, such a configuration already
does not work properly when using AES-CBC since the compressed data is
typically not a multiple of the AES block length in size and EKCD does
not implement any padding scheme. However, EKCD more recently gained
support for using the ChaCha20 cipher, which being a stream cipher does
not have this problem.

Submitted by: sigsys@gmail.com
Reviewed by: cem
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26188