MFC r253095:
Fix typo in comment.

Approved by:	re (hrs)
MFC r251282:
When auto-sizing the buffer cache, limit the amount of physical memory
used as the estimation of size, to 16GB.  This provides around 100K of
buffer headers and corresponding KVA for buffer map at the peak.
Sizing the cache larger is not useful, also resulting in the wasting
and exhausting of KVA for large machines.

MFC note: the commit message was adjusted to match the code change, the
sizing cap is for 16GB, as noted by delphij.

Approved by:	re (delphij)