[INFINIBAND]: Document InfiniBand ioctl use

Add the 0x1b ioctl magic number used by ib_umad module to
Documentation/ioctl-number.txt.
Signed-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/ioctl-number.txt

@@ -72,6 +72,7 @@ Code	Seq#	Include File		Comments
 0x09	all	linux/md.h
 0x12	all	linux/fs.h
 		linux/blkpg.h
+0x1b	all	InfiniBand Subsystem	<http://www.openib.org/>
 0x20	all	drivers/cdrom/cm206.h
 0x22	all	scsi/sg.h
 '#'	00-3F	IEEE 1394 Subsystem	Block for the entire subsystem