IB/uverbs: Move the FD uobj type struct file allocation to alloc_commit

Allocating the struct file during alloc_begin creates this strange
asymmetry with IDR, where the FD has two krefs pointing at it during the
pre-commit phase. In particular this makes the abort process for FD very
strange and confusing.

For instance abort currently calls the type's destroy_object twice, and
the fops release once if abort is done. This is very counter intuitive. No
fops should be called until alloc_commit succeeds, and destroy_object
should only ever be called once.

Moving the struct file allocation to the alloc_commit is now simple, as we
already support failure of rdma_alloc_commit_uobject, with all the
required rollback pieces.

This creates an understandable symmetry with IDR and simplifies/fixes the
abort handling for FD types.

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

include/rdma/uverbs_types.h

@@ -73,7 +73,7 @@ struct uverbs_obj_type_class {
 	 */
 	struct ib_uobject *(*alloc_begin)(const struct uverbs_obj_type *type,
 					  struct ib_uverbs_file *ufile);
-	void (*alloc_commit)(struct ib_uobject *uobj);
+	int (*alloc_commit)(struct ib_uobject *uobj);
 	void (*alloc_abort)(struct ib_uobject *uobj);
 
 	struct ib_uobject *(*lookup_get)(const struct uverbs_obj_type *type,