vm: adjust ifdef for TINY_RCU

There is an ifdef in page_cache_get_speculative() that checks for !SMP
and TREE_RCU, which has been an impossible combination since the advent
of TINY_RCU.  The ifdef enables a fastpath that is valid when preemption
is disabled by rcu_read_lock() in UP systems, which is the case when
TINY_RCU is enabled.  This commit therefore adjusts the ifdef to
generate the fastpath when TINY_RCU is enabled.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reported-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>