sys/vfs/hammer: Ask before reblocking on volume-del

sys/vfs/hammer: Ask before reblocking on volume-del

hammer volume-del finishes instantly if the volume doesn't need
reblocking, but it could take long depending on how much of the
big-blocks within the volume is used.

This commit makes hammer volume-del ask y/n whether to start
reblocking or not if the volume wasn't empty. Using force option
(-F) makes it reblock without asking.

Also note that reblocking doesn't guarantee anything on removing
a volume from filesystem. It just tries to move data to other
volume(s) so the volume may become empty. Thus users should have
choice.

 # hammer volume-del /dev/da2 /HAMMER
 /dev/da2 is not empty, do you want to reblock /dev/da2? [y/n]
 n
 hammer volume-del ioctl: Directory not empty