sys/vfs/hammer: Cleanup ifdef/else in hammer.h [2/2]

sys/vfs/hammer: Cleanup ifdef/else in hammer.h [2/2]

- 872a7eee introduced hammer.h to userspace using ifdef/else switches
  that needs to be cleaned up. It ended up that everything in hammer.h
  was kernel stuff. Userspace needs this header only to include other
  hammer headers that are already available in userspace.

- hunk 1,2 - Cleanup and conform to style(9).

- hunk 3,4 - Userspace doesn't need HAMMER_CHKSPC_XXX. These are only
  internally used by big block allocator.

- hunk 5,6 - Userspace doesn't need hammer_modify_xxx(). These are only
  internally used to begin i/o and generate undo for some meta data.