sbin/hammer: Don't use the same buffer for blockmap layer1 and layer2

sbin/hammer: Don't use the same buffer for blockmap layer1 and layer2

- Functions that lookup blockmap layer1 and layer2 should use
  different buffer variable (set to NULL) for each get_buffer_data()
  call to retrieve ondisk data unless isnew flag is set to 1.

- From the way newfs_hammer allocates layer entries on disks,
  any given layer1 entry and layer2 entry are never both cached in
  a single hammer buffer. Passing layer1 buffer to get_buffer_data()
  as a cache for layer2 happens to be overhead as it takes extra cache
  boundary check (and we know layer2 isn't cached in layer1 buffer).