Improve the way to calculate available pages in tmpfs:
Improve the way to calculate available pages in tmpfs:

 - Don't deduct wired pages from total usable counts because it does not
   make any sense.  To make things worse, on systems where swap size is
   smaller than physical memory and use a lot of wired pages (e.g. ZFS),
   tmpfs can suddenly have free space of 0 because of this;
 - Count cached pages as available; [1]
 - Don't count inactive pages as available, technically we could but that
   might be too aggressive; [1]

[1] Suggested by kib@

MFC after:	1 week

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