generic: inconsistent initial state for test_generic_punch

generic: inconsistent initial state for test_generic_punch

The first test may start with the file from the previous test, and
that is in an unknown state. Hence always remove the test file
before the first test so that it doesn't have extents inside the
test range as it is supposed to be testing into a hole.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>