The global flag remains and is used to prevent duplicated shutdowns and
new io operations after shutdown.
On shutdown, the driver flips the shutdown flag of every pending io
operations and wake them to fail with a shutdown error.
Fixes: #5227
These functions have object safety issues. It also has been decided to
avoid vectored operations on the I/O traits. A later PR will bring back
vectored operations on specific types that support them.
Refs: #2879, #2716
Shuang Li