generic/448: define BASE_TEST_FILE before _cleanup

generic/448: define BASE_TEST_FILE before _cleanup

NFS v4.2 supports SEEK_DATA and SEEK_HOLE, but earlier versions do
not.  As a result, the test exits and runs the cleanup function
without the $BASE_TEST_FILE variable set and the shell expands it to
"rm -f .*", deleting all hidden files in the current directory.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>