IB/rxe: Add a runtime check in alloc_index()

Since index values equal to or above 'range' can trigger memory
corruption, complain if index >= range.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Andrew Boyer <andrew.boyer@dell.com>
Cc: Moni Shoua <monis@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>