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>

common.filter

@@ -214,5 +214,10 @@ _filter_xfs_io_unique()
     common_line_filter | _filter_xfs_io
 }
 
+_filter_scratch()
+{
+	sed -e "s,$SCRATCH_DEV,SCRATCH_DEV,g" -e "s,$SCRATCH_MNT,SCRATCH_MNT,g"
+}
+
 # make sure this script returns success
 /bin/true