RDMA/core: Split gid_attrs related sysfs from add_port()

RDMA/core: Split gid_attrs related sysfs from add_port()

The gid_attrs directory is a dedicated kobj nested under the port,
construct/destruct it with its own pair of functions for
understandability. This is much more readable than having it weirdly
inlined out of order into the add_port() function.

Link: https://lore.kernel.org/r/1c9434111b6770a7aef0e644a88a16eee7e325b8.1623427137.git.leonro@nvidia.com
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>