Specifically, the test was in the wrong place, lacked a cast, didn't
unlock the node, and exited to bad rather than abortit. Now we don't
allow renaming of a file with LINK_MAX references. Move the test to
earlier in the code as it is closer to where ip is obtained, as that
is the style of the rest of the function.
Didn't fix the problems bruce pointed out in the rename man page to
include EMLINK, nor address his complaints about how the whole idea of
incrementing the link count during a rename is potentially asking for
trouble.
Also didn't try to correct potential problem Terry pointed out with
decrements not being similarly protected against underflow.
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