staging: lustre: replace memory_presure funcitons by standard interfaces.

Use memalloc_noreclaim_save() and memalloc_noreclaim_restore(),
and for testing, just directly test the flag in current->flags

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>