clamp kernel dump compression level when using gzip

[vangyzen]

Summary

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.

Test Plan

Here are the vmcore.N.gz sizes for a few configured levels:

-2 30983609 clamped to 1
-1 28151699 in zlib, -1 selects the default, which is 6
 0 31335591 clamped to 1; 0 is valid, but makes no sense
 1 31396046
 9 28064805
10 28010073 clamped to 9