Fix the VM_BCACHE_SIZE_MAX definition on i386 to match the maximal
buffer map size, auto-tuned on the 4GB machine.  Having the maxbcache
bigger than the buffer map causes the transient bio map sizing logic
to assume that there is enough KVA to use approximately 90MB (buffer
map is sized to 110MB, and maxbcache is 200MB).  The increase in the
KVA usage caused other big KVA consumers, like nvidia.ko, to fail the
initialization.

Change the definition for both PAE and non-PAE cases, since PAE is
even more KVA-starved.

Reported and tested by:	David Wolfskill
Discussed with:	alc
Sponsored by:	The FreeBSD Foundation
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