Add support for reporting errors to data waiters via a new
SCOUTFS_IOC_DATA_WAIT_ERR ioctl. This allows waiters to return an error
to readers when staging fails.
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
[zab: renamed to data_wait_err, took ino arg]
Signed-off-by: Zach Brown <zab@versity.com>