xfs: support realtime/log device setup changes in config sections

Currently changing the devices used by "USE_EXTERNAL" environmental
variable is not supported by the config section parsing. Add the
functionality so that we can use config sections to test external
device configs successfully.

This required tracking down a bug in _check_xfs_filesystem() which
was causing a log device to be passed to a test device without an
external log device. This was caused by an uninitialised variable in
the function. I also added full output file removals to the first
couple of generic tests that were failing, because that's where the
check failure output ends up in this case.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/generic/002

@@ -45,6 +45,8 @@ _supported_fs generic
 _supported_os IRIX Linux
 _require_test
 
+rm -f $seqres.full
+
 echo "Silence is goodness ..."
 
 # ensure target directory exists

tests/generic/004

@@ -47,6 +47,8 @@ _supported_os Linux
 _require_test
 _require_xfs_io_command "flink"
 
+rm -f $seqres.full
+
 testfile="${TEST_DIR}/tst-tmpfile-flink"
 
 # test creating a r/w tmpfile, do I/O and link it into the namespace