mm, mempolicy: remove per-process flag

PF_MEMPOLICY is an unnecessary optimization for CONFIG_SLAB users.
There's no significant performance degradation to checking
current->mempolicy rather than current->flags & PF_MEMPOLICY in the
allocation path, especially since this is considered unlikely().

Running TCP_RR with netperf-2.4.5 through localhost on 16 cpu machine with
64GB of memory and without a mempolicy:

	threads		before		after
	16		1249409		1244487
	32		1281786		1246783
	48		1239175		1239138
	64		1244642		1241841
	80		1244346		1248918
	96		1266436		1254316
	112		1307398		1312135
	128		1327607		1326502

Per-process flags are a scarce resource so we should free them up whenever
possible and make them available.  We'll be using it shortly for memcg oom
reserves.

Signed-off-by: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Pekka E...