libdwfl: parse inode in /proc/pid/maps correctly

The inode number in /proc/pid/maps is displayed as "unsigned long"
type.

In one of our x64 system, we have inode number exceeding valid "long"
type range, which caused the following test failure:
   FAIL: dwfl-bug-fd-leak
   FAIL: run-backtrace-dwarf.sh
   FAIL: vdsosyms

The offending map entry:
  7f269246b000-7f269246c000 rw-p 0002e000 00:50 10224326387095067468 /home/...

This patch changed sscanf inode number type from PRIi64 to PRIu64
and fixed the problem.

Signed-off-by: Yonghong Song <yhs@fb.com>