Correct byte-order on LVM metadata reads [Fixes GH#31]

Somehow I managed to completely confuse myself as to the LVM metadata
format, and it was only the bug in htonq/ntohq (GH#6) that I fixed recently
that had made it work all this time.  Once I fixed that bug, this one
appeared.

Thanks to UnwashedMeme for the detective work, and pointing me in the right
direction.

lib/lvm/helpers.rb

@@ -3,16 +3,25 @@ module LVM; end
 module LVM::Helpers
  # Are we on a big-endian system? Needed for our htonq/ntohq methods
  def big_endian?
- @bigendian ||= [1].pack("s") == [1].pack("n")
+ @big_endian ||= [1].pack("s") == [1].pack("n")
  end
 
  def htonq val
  # This won't work on a nUxi byte-order machine, but if you have one of
  # those, I'm guessing you've got bigger problems
- big_endian? ? val : ([val].pack("Q").reverse.unpack("Q").first)
+ big_endian? ? val : swap_longs(val)
  end
 
  def ntohq val
- htonq val
+ big_endian? ? val : swap_longs(val)
+ end
+
+ # On-disk (LVM) format (which is little-endian) to host byte order
+ def dtohq val
+ big_endian? ? swap_longs(val) : val
+ end
+
+ def swap_longs val
+ [val].pack("Q").reverse.unpack("Q").first
  end
 end

lib/lvm/snapshot.rb

@@ -30,15 +30,15 @@ def differences
 
  while in_progress
  # The snapshot on-disk format is a stream of <blocklist>, <blockdata>
- # sets; within each <blocklist>, it's network-byte-order 64-bit block
+ # sets; within each <blocklist>, it's little-endian 64-bit block
  # IDs -- the first is the location (chunk_size * offset) in the origin
  # LV that the data has been changed, the second is the location (again,
  # chunk_size * offset) in the metadata LV where the changed data is
  # being stored.
  (chunk_size / 16).times do
  origin_offset, snap_offset = metafd.read(16).unpack("QQ")
- origin_offset = ntohq(origin_offset)
- snap_offset = ntohq(snap_offset)
+ origin_offset = dtohq(origin_offset)
+ snap_offset = dtohq(snap_offset)
 
  # A snapshot offset of 0 would point back to the metadata
  # device header, so that's clearly invalid -- hence it's the