RDMA/umem: Minor optimizations

RDMA/umem: Minor optimizations

Noticed while reviewing commit d4b4dd1b9706 ("RDMA/umem: Do not use
current->tgid to track the mm_struct") patch.  Why would we take a lock,
adjust a protected variable, drop the lock, and *then* check the input
into our protected variable adjustment?  Then we have to take the lock
again on our error unwind.  Let's just check the input early and skip
taking the locks needlessly if the input isn't valid.

It was also noticed that we set mm = current->mm, we then never modify
mm, but we still go back and reference current->mm a number of times
needlessly.  Be consistent in using the stored reference in mm.

Signed-off-by: Doug Ledford <dledford@redhat.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>