idr_find() may fail and return a NULL pointer. The fix checks the return
value of the function and returns an error in case of NULL.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Acked-by: Michal Kalderon <michal.kalderon@marvell.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>