HAMMER - Fix broken assertion

HAMMER - Fix broken assertion

* An assertion related to validating the flush group was not quite
  correct and causing fsstress panics with nohistory mounts.

* Fixed the assertion to allow a directory's flush group to be different
  from an underlying directory entry.  Directory records can be flushed
  independent of the directory inode itself, particularly under stressful
  conditions where the aggregate flush winds up being too large for the
  buffer cache to handle and has to be split up.

Reported-by: Sascha Wildner <saw@online.de>