ZSTDIO: Correctly initialize zstd context with provided 'level'

Prior to this revision, we allocated sufficient context space for 'level'
but never actually set the compress level parameter, so we would always get
the default '3'.

Reviewed by: markj, vangyzen
MFC after: 12 hours
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D17144