clamp kernel dump compression level when using gzip

If the configured compression level for kernel dumps
it outside the supported range, clamp it to the closest
supported level. Previously, dumpon would fail.

zstd already does this internally, so the compressor
needs no change.

Reviewed by: cem markj
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23765