dlm: <linux/dlm_plock.h> should be "unifdef"ed.

Given that <linux/dlm_plock.h> contains a conditional __KERNEL__ test,
it should be moved from header-y to unifdef-y.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: David Teigland <teigland@redhat.com>