RDMA/srp: Use the attribute group mechanism for sysfs attributes

Simplify the SRP driver by using the attribute group mechanism instead
of calling device_create_file() explicitly.

Link: https://lore.kernel.org/r/20220825213900.864587-5-bvanassche@acm.org


Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Leon Romanovsky <leon@kernel.org>