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In e56f81e0 "dm: refactor ioctl handling" the target specific ioctl
operation was removed in favour of providing a mapping to the underlying
device, to which ioctls are all assumed to apply.  This prevents targets
from having target specific ioctls.

Introduce a new target_ioctl callback which (if present) is offered the
command and arguments for processing.  This callback can return -ENOTTY
to indicate the ioctl should be passed on to the underlying device as
normal.

BugLink: http://bugs.launchpad.net/bugs/1538618

Signed-off-by: Andy Whitcroft <apw@canonical.com>