ibcore: Implement ib_uverbs_get_ucontext_file().

Expose ib_ucontext from a given ib_uverbs_file. Drivers that use the ioctl(9)
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.

Linux commit:
7dc08dcfc8c86cb4457e383734ff6844ddaff876

MFC after: 1 week
Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking