fstests: btrfs, add test for snapshoting after file write + truncate

fstests: btrfs, add test for snapshoting after file write + truncate

Regression test for a btrfs issue where if right after the snapshot
creation ioctl started, a file write followed by a file truncate
happened, with both operations increasing the file's size, the created
snapshot would capture an inconsistent state of the file system tree.
That state reflected the file truncation but it didn't reflect the
write operation, and left a gap between two file extent items (and
that gap corresponded to the total or a partial area of the write
operation's range).

This issue was fixed by the following linux kernel patch:

    Btrfs: fix snapshot inconsistency after a file write followed by truncate

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>