RELENG_4 -> 5 -> 6: significant performance regression

[Bartosz Fabianowski]

> You wrote that Giant is needed in 6.0 and now you write it has been
> removed.

In 4.x, every UFS write requires the Giant lock. In 6.x, Giant is not 
normally required, making file system operations faster. When you enable 
QUOTA, you basically get back to the 4.x behavior where Giant is needed 
for each write. This is why in 6.x UFS will normally be faster but if 
you enable QUOTA, you lose the newly gained performance again.

- Bartosz