[Unionfs] multiple writable branches fix (bug #601)

[Erez Zadok]

Pascal et al,

Regarding bug 601 in

	https://bugzilla.fsl.cs.sunysb.edu/show_bug.cgi?id=601

Please try the following patch:

	https://bugzilla.fsl.cs.sunysb.edu/attachment.cgi?id=196

It should fix the multiple-writable branches problem.  With this patch,
->create, ->symlink, and ->mknod all will try to create new objects in the
first writable branch available (avoiding unnecessary copyup); if no
writable branch can be found, only then unionfs will then try to copyup to
branch 0.

I've done basic testing on the patch and it seems to work.  I'm still
running more extensive testing to ensure that it didn't break anything else
unexpectedly, but those extra tests take many hours; in the mean time I
thought people could try this patch and let me know how it works for you.

Thanks,
Erez.