mm, oom: do not check 0 mask in out_of_memory()

Since commit 60e2793 ("mm, oom: do not trigger out_of_memory from the
#PF"), no user sets gfp_mask to 0.  Remove the 0 mask check and update the
comments.

Link: https://lkml.kernel.org/r/20230508073538.1168-1-haifeng.xu@shopee.com
Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/oom_kill.c

@@ -1130,12 +1130,10 @@ bool out_of_memory(struct oom_control *oc)
 
 	/*
 	 * The OOM killer does not compensate for IO-less reclaim.
-	 * pagefault_out_of_memory lost its gfp context so we have to
-	 * make sure exclude 0 mask - all other users should have at least
-	 * ___GFP_DIRECT_RECLAIM to get here. But mem_cgroup_oom() has to
-	 * invoke the OOM killer even if it is a GFP_NOFS allocation.
+	 * But mem_cgroup_oom() has to invoke the OOM killer even
+	 * if it is a GFP_NOFS allocation.
 	 */
-	if (oc->gfp_mask && !(oc->gfp_mask & __GFP_FS) && !is_memcg_oom(oc))
+	if (!(oc->gfp_mask & __GFP_FS) && !is_memcg_oom(oc))
 		return true;
 
 	/*