dm ioctl: Simplify parameter buffer management code

Merge the two DM_PARAMS_[KV]MALLOC flags into a single flag.

Doing so avoids the crashes seen with previous attempts to consolidate
buffer management to use kvfree() without first flagging that memory had
actually been allocated.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>