sys/vfs/hammer: Add inline functions hammer_modify_buffer|volume_noundo()

sys/vfs/hammer: Add inline functions hammer_modify_buffer|volume_noundo()

- Add noundo wrappers hammer_modify_buffer|volume_noundo() similar to
  the existing inline function hammer_modify_node_noundo() for better
  readability.

- A pair of args (NULL, 0) indicating that it's not generating undo is
  a bit unclear (and there are even comments for them).

- (The compiler doesn't actually inline hammer_modify_node_noundo()
  in my environment, but these one-line wrappers are inlined)