add missing unlock before returning in uvn_detach()

add missing unlock before returning in uvn_detach()

uvn_detach sets UVM_VNODE_RELKILL flag and wait for all async i/o to finish. but
uvm_vnp_terminate() could clear the flag and take over the vnode.

mpi@ noted that this code path is mostly dead code because there is no "async
I/O" (uvn_io() is always synchronous).

ok visa@ mpi@