sizeof(struct g_eli_metadata) will return the exact number of bytes needed
for storing it on the disk.
Without this change GELI was unusable on amd64 (and probably other 64-bit
archs), because sizeof(struct g_eli_metadata) was greater than 512 bytes
and geli(8) was failing on assertion.
Reported by: Michael Reifenberger <mike@Reifenberger.com>
MFC after: 3 days
