xfstests: use a common _filter_scratch function

xfstests: use a common _filter_scratch function

There are a number of tests that use a shell function called
"filter_scratch" or "_filter_scratch" in order to have the actual
scratch device or mount point show up in test output with a symbolic
name.  There are two sets, each following a slightly different
convention.  Put a common _filter_scratch function definition in
"common.filter" and have all test scripts use that instead.
Choosing one of the two conventions meant that a few test output
files had to be changed.

In addition, add a call to _filter_scratch to test 185, and update
its output accordingly.

Signed-off-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>