ffs_syncvnode(): avoid a LoR for SU

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
Reviewed by: mckusick
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D37997