arm64/crashdump: unify routine to get page_offset

There are two funcs to get page_offset:
  get_kernel_page_offset()
  get_page_offset()

Since get_kernel_page_offset() does not observe the kernel formula, and
remove it. Unify them in order to introduce 52-bits VA kernel more
easily in the coming patch.

Signed-off-by: Pingfan Liu <piliu@redhat.com>
Reviewed-by: Philipp Rudo <prudo@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>