perf tools: Use __map__is_kernel() when synthesizing kernel module mmap records

Equivalent and removes one more case of using dso->kernel.

  # perf record -a usleep 1
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.768 MB perf.data (30 samples) ]

Before:

  [root@zoo ~]# perf script --show-task --show-mmap | head -3
   swapper 0 [0] 0.0: PERF_RECORD_MMAP -1/0: [0xffffffff81000000(0x1f000000) @ 0xffffffff81000000]: x [kernel.kallsyms]_text
   swapper 0 [0] 0.0: PERF_RECORD_MMAP -1/0: [0xffffffffa0000000(0xa000) @ 0]: x /lib/modules/4.3.0-rc1+/kernel/drivers/acpi/video.ko
   swapper 0 [0] 0.0: PERF_RECORD_MMAP -1/0: [0xffffffffa000a000(0x5000) @ 0]: x /lib/modules/4.3.0-rc1+/kernel/drivers/i2c/algos/i2c-algo-bit.ko
  #

  # perf script --show-task --show-mmap | head -3
   swapper 0 [0] 0.0: PERF_RECORD_MMAP -1/0: [0xffffffff81000000(0x1f000000) @ 0xffffffff81000000]: x [kernel.kallsyms]_text
   swapper 0 [0] 0.0: PERF_RECORD_MMAP -1/0: [0xffffffffa0000000(0xa000) @ ...

tools/perf/util/event.c

@@ -378,7 +378,7 @@ int perf_event__synthesize_modules(struct perf_tool *tool,
 	for (pos = maps__first(maps); pos; pos = map__next(pos)) {
 		size_t size;
 
-		if (pos->dso->kernel)
+		if (__map__is_kernel(pos))
 			continue;
 
 		size = PERF_ALIGN(pos->dso->long_name_len + 1, sizeof(u64));