xfstests: fix common source file path

After the re-factor, common.* have been renamed to common/* but there
are several files still look for the old path, fix it.

Signed-off-by: Eryu Guan <eguan@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>

README

@@ -65,7 +65,7 @@ Preparing system for tests (IRIX and Linux):
                environment variable set to "yes" will enable their use.
              - setenv DIFF_LENGTH "number of diff lines to print from a failed test",
                by default 10, set to 0 to print the full diff
-        - or add a case to the switch in common.config assigning
+        - or add a case to the switch in common/config assigning
           these variables based on the hostname of your test
           machine
 	- or add these variables to a file called local.config and keep that
@@ -113,7 +113,7 @@ Test script environment:
 
     When developing a new test script keep the following things in
     mind.  All of the environment variables and shell procedures are
-    available to the script once the "common.rc" file has been
+    available to the script once the "common/rc" file has been
     sourced.
 
      1. The tests are run from an arbitrary directory.  If you want to
@@ -166,7 +166,7 @@ Test script environment:
 	_within_tolerance	- fancy numerical "close enough is good
 				  enough" filter for deterministic
 				  output ... see comments in
-				  common.filter for an explanation
+				  common/filter for an explanation
 
 	_filter_date		- turn ctime(3) format dates into the
 				  string DATE for deterministic