dump: clamp guest-provided mapping lengths to ramblock sizes

Even a trusted & clean-state guest can map more memory than what it was
given. Since the vmcore contains RAMBlocks, mapping sizes should be
clamped to RAMBlock sizes. Otherwise such oversized mappings can exceed
the entire file size, and ELF parsers might refuse even the valid portion
of the PT_LOAD entry.

Related RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=981582

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
