bpf: add bpffs pretty print for percpu arraymap/hash/lru_hash

bpf: add bpffs pretty print for percpu arraymap/hash/lru_hash

Added bpffs pretty print for percpu arraymap, percpu hashmap
and percpu lru hashmap.

For each map <key, value> pair, the format is:
   <key_value>: {
	cpu0: <value_on_cpu0>
	cpu1: <value_on_cpu1>
	...
	cpun: <value_on_cpun>
   }

For example, on my VM, there are 4 cpus, and
for test_btf test in the next patch:
   cat /sys/fs/bpf/pprint_test_percpu_hash

You may get:
   ...
   43602: {
	cpu0: {43602,0,-43602,0x3,0xaa52,0x3,{43602|[82,170,0,0,0,0,0,0]},ENUM_TWO}
	cpu1: {43602,0,-43602,0x3,0xaa52,0x3,{43602|[82,170,0,0,0,0,0,0]},ENUM_TWO}
	cpu2: {43602,0,-43602,0x3,0xaa52,0x3,{43602|[82,170,0,0,0,0,0,0]},ENUM_TWO}
	cpu3: {43602,0,-43602,0x3,0xaa52,0x3,{43602|[82,170,0,0,0,0,0,0]},ENUM_TWO}
   }
   72847: {
	cpu0: {72847,0,-72847,0x3,0x11c8f,0x3,{72847|[143,28,1,0,0,0,0,0]},ENUM_THREE}
	cpu1: {72847,0,-72847,0x3,0x11c8f,0x3,{72847|[143,28,1,0,0,0,0,0]},ENUM_THREE}
	cpu2: {72847,0,-72847,0x3,0x11c8f,0x3,{72847|[143,28,1,0,0,0,0,0]},ENUM_THREE}
	cpu3: {72847,0,-72847,0x3,0x11c8f,0x3,{72847|[143,28,1,0,0,0,0,0]},ENUM_THREE}
   }
   ...

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>