generic: add fiemap test that does prealloc

generic: add fiemap test that does prealloc

I noticed that btrfs wasn't setting unwritten on prealloc test, and then
subsequently noticed that we weren't testing fiemap on prealloc extents with the
fiemap-tester.  This patch adds another test that does the same as generic/225
only with prealloc enabled.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>