xfstests: use value of FSTYP if defined externally

The initial value of FSTYP is unconditionally set to 'xfs' and the
filesystem type is taken from the TEST_DEV. This could lead to confusion
if the device hasn't been formatted yet, eg. an empty image in VM, or
a different test setup took place before.

Now one can specify the desired FSTYP in advance and be safe. If unset,
the fallback to TEST_DEV type continues to work.

Signed-off-by: David Sterba <dsterba@suse.cz>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

README

@@ -65,6 +65,9 @@ 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
+             - setenv FSTYP "the filesystem you want to test", the filesystem
+               type is devised from the TEST_DEV device, but you may want to
+               override it; if unset, the default is 'xfs'
         - or add a case to the switch in common/config assigning
           these variables based on the hostname of your test
           machine