Fix information leak in geli(8) integrity mode

In integrity mode, a larger logical sector (e.g., 4096 bytes) spans several
physical sectors (e.g., 512 bytes) on the backing device. Due to hash
overhead, a 4096 byte logical sector takes 8.5625 512-byte physical sectors.
This means that only 288 bytes (256 data + 32 hash) of the last 512 byte
sector are used.

The memory allocation used to store the encrypted data to be written to the
physical sectors comes from malloc(9) and does not use M_ZERO.

Previously, nothing initialized the final physical sector backing each
logical sector, aside from the hash + encrypted data portion. So 224 bytes
of kernel heap memory was leaked to every block :-(.

This patch addresses the issue by initializing the trailing portion of the
physical sector in every logical sector to zeros before use. A much simpler
but higher overhead fix would be to tag the entire allocation M_ZERO.

PR: 222077
Reported by: Maxim Khitrov <max AT mxcrypt.com>
Reviewed by: emaste
Security: yes
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D12272