dm ioctl: optimize functions without variable params

Device-mapper ioctls receive and send data in a buffer supplied
by userspace.  The buffer has two parts.  The first part contains
a 'struct dm_ioctl' and has a fixed size.  The second part depends
on the ioctl and has a variable size.

This patch recognises the specific ioctls that do not use the variable
part of the buffer and skips allocating memory for it.

In particular, when a device is suspended and a resume ioctl is sent,
this now avoid memory allocation completely.

The variable "struct dm_ioctl tmp" is moved from the function
copy_params to its caller ctl_ioctl and renamed to param_kernel.
It is used directly when the ioctl function doesn't need any arguments.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>