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

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