perf symbols: Fix symbols searching for module in buildid-cache

Before this patch, if a sample is triggered inside a module not in
/lib/modules/`uname -r`/, even if the module is in buildid-cache, 'perf
report' will still be unable to find the correct symbol.  For example:

  # rm -rf ~/.debug/
  # perf buildid-cache -a ./mymodule.ko
  # perf probe -m ./mymodule.ko -a get_mymodule_val
  Added new event:
    probe:get_mymodule_val (on get_mymodule_val in mymodule)

  You can now use it in all perf tools, such as:

 	perf record -e probe:get_mymodule_val -aR sleep 1

  # perf record -e probe:get_mymodule_val cat /proc/mymodule
  mymodule:3
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.011 MB perf.data (1 samples) ]

  # perf report --stdio
  [SNIP]
  #
  # Overhead  Command  Shared Object     Symbol
  # ........  .......  ................  ......................
  #
    100.00%  cat      [mymodule]        [k] 0x0000000000000001

  # perf report -vvvv --stdio
  dso__load_sym: adjusting symbol: st_value: 0 sh_addr: 0 sh_offset: 0x70
  symbol__new: get_mymodule_val 0x70-0x8a
  [SNIP]

This is caused by dso__load() -> dso__load_sym(). In dso__load(), kmod
is true only when its file is found in some well know directories. All
files loaded from buildid-cache are treated as user programs. Following
dso__load_sym() set map->pgoff incorrectly.

This patch gives kernel modules in buildid-cache a chance to adjust
value of kmod. After dso__load() get the type of symbols, if it is
buildid, check the last 3 chars of original filename against '.ko', and
adjust the value of kmod if the file is a kernel module.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Cody P Schafer <dev@codyps.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Jeremie Galarneau <jeremie.galarneau@efficios.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kirill Smelkov <kirr@nexedi.com>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1454680939-24963-3-git-send-email-wangnan0@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>