Add a default value for VM_BCACHE_SIZE_MAX of 400MB.  This is copied from amd64, and is a factor of 3 less than the value previously auto-sized on a 12GB machine, which would cause an overflow in calculations involving the maxbcache int, causing bufinit() to loop forever at boot.
Add a default value for VM_BCACHE_SIZE_MAX of 400MB.  This is copied from
amd64, and is a factor of 3 less than the value previously auto-sized on
a 12GB machine, which would cause an overflow in calculations involving the
maxbcache int, causing bufinit() to loop forever at boot.

Reviewed by:	mlaier, peter

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