btrfs/271: use the common fail_request setup helpers

btrfs/271: use the common fail_request setup helpers

Use the helpers from common/fail_make_request instead of open coding
them.  This switches to using a higher error count than the existing
code, which was the intention from the very beginning (and doesn't
actuallt matter for the short sequences in this test).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>