btrfs: add test for multiple fsync with adjacent preallocated extents

btrfs: add test for multiple fsync with adjacent preallocated extents

Test a scenario where we do several partial writes into multiple
preallocated extents across two transactions and with several fsyncs
in between. The goal is to check that the fsyncs succeed.

Currently the last fsync fails with an -EIO error, and it aborts the
current transaction. This issue is fixed by a patch with the following
subject:

  "btrfs: fix fsync failure and transaction abort after writes to prealloc extents"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>