IB/uverbs: Make the write path destroy methods use the same flow as ioctl

The ridiculous dance with uobj_remove_commit() is not needed, the write
path can follow the same flow as ioctl - lock and destroy the HW object
then use the data left over in the uobject to form the response to
userspace.

Two helpers are introduced to make this flow straightforward for the
caller.

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