uring: always pair io_uring_get_probe_ring and io_uring_free_probe

uring: always pair io_uring_get_probe_ring and io_uring_free_probe

As of liburing 2.4-to-be, liburing has ceased using malloc for its
probe allocations: it's concealed behind layers of confusing macros,
but io_uring_get_probe_ring() doesn't actually use libc's malloc
but ratehr its own __uring_alloc/__uring_free functions.  So it no
longer works to free() the return value of io_uring_get_probe_ring():
you have to use io_uring_free_probe() instead.

I am going to report this to liburing upstream as a possible bug,
but it is easy to work around in plocate in any case.