sys/geom/eli: Switch bzero() to explicit_bzero() for sensitive data

In GELI, anywhere we are zeroing out possibly sensitive data, like
the metadata struct, the metadata sector (both contain the encrypted
master key), the user key, or the master key, use explicit_bzero.

Didn't touch the bzero() used to initialize structs.

Reviewed by: delphij, oshogbo
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D9809