emulation lookups.
If doing a lookup relative to the emulation root, prepend the emulation root
to the traced filename.
While here pass the filename length through to the ktrace code since namei()
knows the length and ktr_namei() would have to call strlen().
Note: that if namei() is being called during execve processing, the emulation
root name isn't available and "/emul/???" is used. Also namei() has to use
strlen() to get the lenght on the emulatoon root - even though it is a
compile-time constant string.
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