xfstests: generic: add test for double msync, motivated by a btrfs bug
This test is motivated by a btrfs issue where a ranged fsync would
prevent a subsequent fsync from persisting any extents that were
dirty at the time of the first fsync but that were outside the range
of that first fsync (which should have been persisted by the second
fsync).
This bug in btrfs is fixed by the following linux kernel patch:
Btrfs: fix fsync data loss after a ranged fsync
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>