[PATCH] x86_64: fix an unnecessary message with --mem-usage option

commit bc8b3bbf ("arm64: restore info->page_offset and implement
paddr_to_vaddr_arm64()") added get_phys_base() to show_mem_usage(),
but at this time there is no vmcoreinfo yet, so get_phys_base_x86_64()
executes SYMBOL_INIT() and prints the following message.

  # makedumpfile --mem-usage /proc/kcore
  init_dwarf_info: Can't find absolute path to debuginfo file.
  ...

This patch adds the check if vmlinux is specified, and suppress the
message.

Signed-off-by: Kazuhito Hagio <k-hagio@ab.jp.nec.com>