Add GetBase support for kernel mappings as reported by Linux Perf

Linux Perf reports kernel mappings with an offset == start.
Currently GetBase() cannot handle such a case.

This behavior can be seen from a perf.report data with samples in the kernel:

$ sudo perf report -D perf.data | grep MMAP
0 0x108 [0x50]: PERF_RECORD_MMAP -1/0: [0xffffffffaf800000(0x1087a000) @ 0xffffffffaf800000]: x [kernel.kallsyms]_text