FFS: implement special VOP_VPUT_PAIR().

It cleans IN_NEEDSYNC flag on dvp before returning, by applying
ffs_syncvnode() until success or an error different from ERELOOKUP.
IN_NEEDSYNC cleanup is required to avoid creating holes in the directories
when extended into indirect block.

Reviewed by: chs, mckusick
Tested by: pho
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation