Adapt dholland@'s fix to ufs_rename to fix PR kern/43582. Address several
other MP locking issues discovered during the course of investigating the
same problem.
Removed extraneous vn_lock() calls on the Ifile, since the Ifile writes
are controlled by the segment lock.
Fix PR kern/45982 by deemphasizing the estimate of how much metadata
will fill the empty space on disk when the disk is nearly empty
(t_renamerace crates a lot of inode blocks on a tiny empty disk).
ed08fe6512