RDMA/srp: Fix a recently introduced memory leak

Only allocate a memory registration list if it will be used and if it will
be freed.

Link: https://lore.kernel.org/r/20210524041211.9480-5-bvanassche@acm.org


Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Fixes: f273ad4f ("RDMA/srp: Remove support for FMR memory registration")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>