amd64: reduce VM_KMEM_SIZE_SCALE to 1 allowing kernel to use more memory
amd64: reduce VM_KMEM_SIZE_SCALE to 1 allowing kernel to use more memory

KVA space is abundant on amd64, so there is no reason to limit kernel
map size to a fraction of available physical memory.  In fact, it could
be larger than physical memory.

This should help with memory auto-tuning for ZFS and shouldn't affect
other workloads.
This should reduce number of circumstances for "kmem_map too small"
panics, but probably won't eliminate them entirely due to potential kmem
fragmentation.

In fact, you might want/need to limit maximum ARC size after this commit
if you need to resrve more memory for applications.

This change was discussed on arch@ and nobody said "don't do it".

MFC after:	6 weeks

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