HAMMER VFS - Minor bug (caught by assertion panic)

HAMMER VFS - Minor bug (caught by assertion panic)

* A snapshot can sometimes contain visible inodes whos nlinks count is 0,
  essentially the snapshot 'catches' the file in the middle of being deleted.

* HAMMER was attempting to truncate the data for such inodes if the file
  were opened and then closed, and failed to check whether the inode was a
  snapshot or a current inode.  This flowed through until it hit an assertion
  designed to detect precisely that case.

* Fixed by adding a check to determine if the inode is a snapshot and/or
  the filesystem is mounted read-only.

Repored-by: Max <herrgard@gmail.com>