ffs: call ufsdirhash_dirtrunc() right after setting directory size

Later processing of ffs_truncate() might temporary unlock the directory
vnode, causing unsychronized dirhash and inode sizes if update is
postponed to UFS_TRUNCATE() callers.

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