dtrace: ensure DTrace can use get_user_pages safely

dtrace: ensure DTrace can use get_user_pages safely

The processing of the DTrace-specific FOLL_IMMED flag was not robust
enough.  We could still get into a situation where cond_resched() was
called (which is bad) or where the VMA area would get extended (which
is also bad).  The only code that passes this flag is DTrace support
code, and when the flag is not passed, the execution flow is not at all
affected.

Orabug: 25640153
Signed-off-by: Kris Van Hees <kris.van.hees@oracle.com>
Reviewed-by: Chuck Anderson <chuck.anderson@oracle.com>
Reviewed-by: Nick Alcock <nick.alcock@oracle.com>
Reviewed-by: Tomas Jedlicka <tomas.jedlicka@oracle.com>