MAINTAINERS: Update SRP entries

Reflect the acquisition of SanDisk by Western Digital in my e-mail
address. Remove the reference to David Dillow's git tree since SRP patches
are queued by Doug and Jason. Remove the reference to the OpenFabrics
website since the srp_daemon source code has been moved from that website
into the rdma-core project. Add an entry for the SRP target driver.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: David Dillow <dillow@google.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>