dtrace: ensure one can try to get user pages without locking or faulting

dtrace: ensure one can try to get user pages without locking or faulting

This commit changes the FOLL_NOFAULT flag into a FOLL_IMMED flag, to
more accurately convey its meaning, i.e. to request user pages without
waiting for any locks and without servicing any page faults as a result
of the request.  This is necessary in order to request user pages from
interrupt context.

This also completes the implementation by ensuring that the PTE spinlock
is checked rather than trying to lock it (and possibly get stuck in a
deadlock spinning for it).

Orabug: 18653713

Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Acked-by: Chuck Anderson <chuck.anderson@oracle.com>