generic: test for truncating a file into the middle of a hole

generic: test for truncating a file into the middle of a hole

Test that after truncating a file into the middle of a hole causes the
new size of the file to be persisted after a clean unmount of the
filesystem (or after the inode is evicted). This is for the case where
all the data following the hole is not yet durably persisted, that is,
that data is only present in the page cache.

This test is motivated by an issue found in btrfs, which got fixed by
the patch titled:

  "Btrfs: fix shrinking truncate when the no_holes feature is enabled"

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