IB/uverbs: Tidy up remaining references to ucontext

Unnecessary clutter, to indirect through ucontext when the ufile would do.
Generally most of the code code should only be working with ufile, except
for a few places that touch the driver interface.

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