IB/mlx5: Declare local functions 'static'

This patch avoids that sparse complains about missing function
declarations.

Fixes: c9990ab3 ("RDMA/umem: Move all the ODP related stuff out of ucontext and into per_mm")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>