IB/core: Expose ib_ucontext from a given ib_uverbs_file

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