dumpon: skip size check if using zstd

As with gzip, let the dump device be smaller than physical memory
when using zstd and full dumps.

Also print the error message if the size check fails, even if -v
is not specified. Failing silently is not friendly.

Reviewed by: cem markj
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23923