This is like DMU_TX_ASSIGN_NOSUSPEND, but only when failmode=continue,
and returning EIO if the pool is suspended. Its designed to be easy to
use from syscalls and similar without the ceremony of checking the for
EAGAIN and failmode every time.
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
(cherry picked from commit 6bed8644dd2afa0e39727e9e90642479c2416521)