[PATCH v3] when refiltering, initialize refiltered bitmap2 from the kdump file's bitmap2

When refiltering on kdump format file, there is no info about pt_load[] for
exclude_nodata_pages(), and also we can not expect more data than the kdump
file can provide, hence this patch suggests to initialize the refiltered
bitmap2 from the kdump file's bitmap2. As for the statistic of pfn_memhole,
it should be calculated and discount the bits in kdump file's bitmap1.

Note about the bug reported by the following ops:
  makedumpfile -l --message-level 1 -d 31 /proc/vmcore /path/to/vmcore
  makedumpfile  --split  -d 31 ./vmcore dumpfile_{1,2,3} 2>&1
And get the following error:
  Excluding unnecessary pages                       : [100.0 %] \
  readpage_kdump_compressed: pfn(9b) is excluded from /var/crash/127.0.0.1-2018-07-02-22:10:38/vmcore.
  readmem: type_addr: 1, addr:9b000, size:4096
  read_pfn: Can't get the page data.
  writeout_multiple_dumpfiles: Child process(2277) finished incompletely.(256)
  Copying data                                      : [ 24.6 %] -           eta: 2s
  makedumpfile Failed.

Cc: Kazuhito Hagio <k-hagio@ab.jp.nec.com>
Signed-off-by: Pingfan Liu <piliu@redhat.com>