RDMA/core: Optimize XRC target lookup

Replace the mutex with read write semaphore and use xarray instead of
linked list for XRC target QPs. This will give faster XRC target
lookup. In addition, when QP is closed, don't insert it back to the xarray
if the destroy command failed.

Link: https://lore.kernel.org/r/20200706122716.647338-4-leon@kernel.org
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>