MAINTAINERS: Add entry for iSCSI Extensions for RDMA (iSER) initiator

Add entry for the iSER initiator driver and which is maintained by Or
Gerlitz and Roi Dayan below the kernel InfiniBand subsystem.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>