btrfs: add test for seeing unseen fsync errors on newly open files

btrfs: add test for seeing unseen fsync errors on newly open files

This adds a regression test for the following kernel patch:

    b4678df184b3 ("errseq: Always report a writeback error once")

This is motivated by some rather odd behavior done by the PostgreSQL
project. The main database writers will offload the fsync calls to a
separate process, which can open files after a writeback error has
already occurred.

This used to work with older kernels that reported the error to only
one fd, but with the errseq_t changes we lost the ability to see
errors that occurred before the open. The above patch restores that
behavior.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>