hammer2 - Implement 'growfs' directive

hammer2 - Implement 'growfs' directive

* Implement the 'growfs' directive, as in:

  hammer2 growfs /mnt

* If you resize the partition a hammer2 filesystem resides on, you
  can grow the filesystem into the partition via this command.

  The hammer2 filesystem must be mounted to issue this command.

* Note that disklabel partitions can be made larger in-place, even
  with the filesystem using the partition already mounted, as long
  as the drive actually has room to spare.