generic: add a test for fallocate+truncate

generic: add a test for fallocate+truncate

There was some confused about what the fs was supposed to do when you truncate
at i_size with preallocated space past i_size.  We decided on the following
things

1) truncate(i_size) will trim all blocks past i_size.
2) truncate(x) where x > i_size will not trim all blocks past i_size.

This test is to make sure we're all acting sanely.  Thanks,

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