generic: test for fsync after file truncations

generic: test for fsync after file truncations

Test that if we truncate a file to a smaller size, then truncate it to
its original size or a larger size, then fsyncing it and a power failure
happens, the file will have the range [first_truncate_size, last_size[
with all bytes having a value of 0x00 if we read it the next time the
filesystem is mounted.

This test is motivated by a bug found in btrfs, which is fixed by a patch
titled: "Btrfs: fix fsync after truncate when no_holes feature is enabled"

Tested against ext3/4, xfs, btrfs (with and without the fix, and with the
no_holes feature disabled), f2fs, reiserfs and nilfs2.

All filesystems pass the test except for unpatched btrfs with the
no_holes feature enabled (as expected) and f2fs. Both produce the
following file contents that differ from the golden output:

  File foo content after log replay:
  0000000 aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
  *
  0200000 bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb bb
  *
  0372000
  File bar content after log replay:
  0000000 ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee ee
  *
  0200000 ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
  *
  0372000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  *
  0772000

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>