xfs/015: keep create_file running until growfs finished

xfs/015: keep create_file running until growfs finished

create_file may run over before growfs, which depends on many reasons. such as
the schedule algorithm, the workload of testing machine, etc. we should always
make sure the create_file run over after growfs, then we can get the valid
result of this test.

Signed-off-by: George Wang <xuw2015@gmail.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>