RDMA/core: Allocate the pkey cache only if the pkey_tbl_len is set

Allocate the pkey cache only if the pkey_tbl_len is set by the provider,
also add checks to avoid accessing the pkey cache when it not initialized.

Link: https://lore.kernel.org/r/20200714183414.61069-3-kamalheib1@gmail.com


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