On i386, double the default size of the bio transient map.  With the maxbcache size fixed, the auto-tuned transient map is too small for real-world load on i386.
On i386, double the default size of the bio transient map.  With the
maxbcache size fixed, the auto-tuned transient map is too small for
real-world load on i386.

Tested by:	David Wolfskill
Sponsored by:	The FreeBSD Foundation

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