RDMA/uverbs: Support external FD uobjects

RDMA/uverbs: Support external FD uobjects

Add support for uobjects that wrap externally allocated file
descriptors (FDs).

In this mode, the FD number still follows the standard uverbs allocation
flow, but the file pointer is allocated externally and has its own fops
and private data.

As a result, alloc_begin_fd_uobject() must handle cases where
fd_type->fops is NULL, and both alloc_commit_fd_uobject() and
alloc_abort_fd_uobject() must account for whether filp->private_data
exists, since it is populated outside the standard uverbs flow.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Link: https://patch.msgid.link/20260201-dmabuf-export-v3-1-da238b614fe3@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>