generic: test mounting filesystem after fsync of a tmpfile

generic: test mounting filesystem after fsync of a tmpfile

Test that if we fsync a tmpfile, without adding a hard link to it, and
then power fail, we will be able to mount the filesystem without
triggering any crashes, warnings or corruptions.

This test is motivated by an issue in btrfs where this scenario triggered
a warning (without any side effects). The following linux kernel patch
fixes the issue in btrfs:

  "Btrfs: fix warning when replaying log after fsync of a tmpfile"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>