Drivers that use the IOCTL API may have the ib_uverbs_file and need a
way to get the related ib_ucontext from it, this is enabled by this
patch.

Downstream patches from this series will use it.
Signed-off-by: NYishai Hadas <yishaih@mellanox.com>
Signed-off-by: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>