IB/mlx5: Use the ucontext from the uobj, not the file

This approach matches the standard flow of the typical write method that
relies on the HW object to store the device and the uobject to access the
ucontext.  Avoids the use of the devx_ufile2uctx in several places will
make revising the semantics of ib_uverbs_get_ucontext() in the next patch
simpler.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>