RDMA/core: Expose pkeys sysfs files only if pkey_tbl_len is set

Expose the pkeys sysfs files only if the pkey_tbl_len is set by the
providers.

Link: https://lore.kernel.org/r/20200714183414.61069-2-kamalheib1@gmail.com


Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>