1) The OPW() function macro should have the same return type like the
function it executes.
2) The DEVFS I/O-limit should be enforced for all character device reads
and writes.
3) The character device file handle should be passable, same as for
DEVFS based file handles.
Reported by: jbeich @
MFC after: 1 week
Sponsored by: Mellanox Technologies
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