Grabbing the spinlock for every bucket even if it's empty, was causing
significant perfomance cost when traversing htab maps that have only a
few entries. This patch addresses the issue by checking first the
bucket_cnt, if the bucket has some entries then we go and grab the
spinlock and proceed with the batching.

Tested with a htab of size 50K and different value of populated entries.

Before:
  Benchmark             Time(ns)        CPU(ns)
  ---------------------------------------------
  BM_DumpHashMap/1       2759655        2752033
  BM_DumpHashMap/10      2933722        2930825
  BM_DumpHashMap/200     3171680        3170265
  BM_DumpHashMap/500     3639607        3635511
  BM_DumpHashMap/1000    4369008        4364981
  BM_DumpHashMap/5k     11171919       11134028
  BM_DumpHashMap/20k    69150080       69033496
  BM_DumpHashMap/39k   190501036      190226162

After:
  Benchmark             Time(ns)        CPU(ns)
  ---------------------------------------------
  BM_DumpHashMap/1        202707         200109
  BM_DumpHashMap/10       213441         210569
  BM_DumpHashMap/200      478641         472350
  BM_DumpHashMap/500      980061         967102
  BM_DumpHashMap/1000    1863835        1839575
  BM_DumpHashMap/5k      8961836        8902540
  BM_DumpHashMap/20k    69761497       69322756
  BM_DumpHashMap/39k   187437830      186551111

Fixes: 05799638 ("bpf: Add batch ops to all htab bpf map")
Signed-off-by: Brian Vazquez <brianvv@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200218172552.215077-1-brianvv@google.com