fstests: generic test for fsync after hole punching

fstests: generic test for fsync after hole punching

Test that a file fsync works after punching a hole for the same file
range multiple times, and that after log/journal replay the file's
content and layout are correct.

This test is motivated by a bug found in btrfs, which is fixed by
the following linux kernel patch:

  "Btrfs: fix hole punching when using the no-holes feature"

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