When externding directory inode in ufs_direnter(), adjust i_endoff.

This change is formally not needed, since i_endoff not used in all
code paths after the call to ufs_direnter(), and i_endoff is
recalculated by the next lookup. But having the value correct makes
the reasoning about code simpler.

Reported and tested by: pho
Reviewed by: mckusick
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks