RDMA/mlx5: Always remove MRs from the cache before destroying them

The cache bucket tracks the total number of MRs that exists, both inside
and outside of the cache. Removing a MR from the cache (by setting
cache_ent to NULL) without updating total_mrs will cause the tracking to
leak and be inflated.

Further fix the rereg_mr path to always destroy the MR. reg_create will
always overwrite all the MR data in mlx5_ib_mr, so the MR must be
completely destroyed, in all cases, before this function can be
called. Detach the MR from the cache and unconditionally destroy it to
avoid leaking HW mkeys.

Fixes: afd14174 ("IB/mlx5: Use direct mkey destroy command upon UMR unreg failure")
Fixes: 56e11d62

 ("IB/mlx5: Added support for re-registration of MRs")
Link: https://lore.kernel.org/r/20200310082238.239865-8-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>