Perhaps it's better to think of it another way - Pain is just your sense of touch. It's "Painful" when you have an overwhelming amount of information on what you are "touching", but a light, soft touch or certain types of patterns of touch (soft, fluffy things) are pleasant.
Being overwhelmed by pain is being hit with far too much sensory information for your brain to properly process, especially when there are reflexive reactions to sensory input like touch/pain.
Hence, I'm going to have to agree with Neonivek, here - "negative pain" makes little sense - it's like "unseeing" something. (Once you have seen, you cannot unsee.) You can be blinded by seeing too much light, but darkness isn't anti-light, it's an absence of light. Likewise, pleasant forms of light are just moderate amounts of light that convey data we enjoy. Gaining pleasure from something we see isn't the same as having negative amounts of blinding concentrations of light.
Having something that functionally performs a "pain tolerance" level, the point at which pain causes a mind to no longer function properly makes sense. If we are dealing with berserkers, what we need is a different threshold - they still feel pain, but they will go berserk as a response to their pain, but can still be overwhelmed by pain if they go too far beyond their limits, themselves.
We already have berserking dwarves, and they aren't running on "negative pain".
Rather, I would think that there could be something that is trainable about pain resistance, as well. (Leaving danger rooms to "train dwarves to take more pain" aside...) Making dwarves who are more experienced with combat generally more pain-resistant would make sense. There are plenty of other jobs that would probably "toughen you up" to pain, as well, but it would be a good start to have a grizzled veteran of combat better trained and able to set aside the reflexive reaction to being injured in his/her mind.