RDMA/uverbs: Declare closing variable as boolean

The "closing" variable is used as boolean and set to "true" in one
place, update the declaration of that variable and their other
assignment to proper type.

Fixes: e951747a ("IB/uverbs: Rework the locking for cleaning up the ucontext")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>