Added a tunable for spa_get_asize

This function is used to estimate the actual disk consumption from the
size of data being written, and it does this conservatively (so as
not to run over available space and quota, IIUC.)

The problem is that this estimate is very very conservative, at whopping
24 times the typical data size. Many multiplication factors come from
features that some may not activate, such as RAIDZ and dedup.

I started a conversation about this in open-zfs mailing list
(http://lists.open-zfs.org:8080/pipermail/developer/2013-November/000360.html),
but I expect some of the changes to be above my expertise level.

In the mean time, a much easier thing to do is to turn this
multiplication factor into a tunable, so that administrators can set
this value manually.

(The background for this change is that I was investigating why my TXG
size is small, and it turns out that my 100MB write to 2GB storage was
hitting the quota ceiling thanks to the x24 conservative estimate.)

Signed-Off-By: Kohsuke Kawaguchi <kk@kohsuke.org>