IB/mad: Agent registration is process context only

Document this (it's implicitly true due to sleeping operations already
in use in both registration and deregistration).  Use this fact to use
spin_lock_irq instead of spin_lock_irqsave.  This improves performance
slightly.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>