IB/rdmavt: Always return ERR_PTR from rvt_create_mmap_info()

IB/rdmavt: Always return ERR_PTR from rvt_create_mmap_info()

The commit below modified rvt_create_mmap_info() to return ERR_PTR's but
didn't update the callers to handle them. Modify rvt_create_mmap_info() to
only return ERR_PTR and fix all error checking after
rvt_create_mmap_info() was called.

Fixes: a8101ca5302f ("IB: Pass only ib_udata in function prototypes")
Link: https://lore.kernel.org/r/20200424173146.10970-1-sudipm.mukherjee@gmail.com
Cc: stable@vger.kernel.org [5.4+]
Tested-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Acked-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>