Fix some recent regressions after r328436 in the LinuxKPI:

Fix some recent regressions after r328436 in the LinuxKPI:

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