ffs_syncvnode(): avoid a LoR for SU

[kib]

Summary

There is another case where SU code does ffs_syncvnode(dvp) for the
parent directory dvp while the child vnode vp is locked.  Avoid the
issue by relocking and returning ERELOOKUP to indicate the need of
resync.

Reported by:    jkim
Tested by:      pho

[mckusick]

This change looks correct.

After rename, fsync when running with soft updates has to be the most complex code in the filesystem.