libbpf: Fixes incorrect rx_ring_setup_done

When calling xsk_socket__create_shared(), the logic at line 1097 marks a
boolean flag true within the xsk_umem structure to track setup progress
in order to support multiple calls to the function.  However, instead of
marking umem->tx_ring_setup_done, the code incorrectly sets
umem->rx_ring_setup_done.  This leads to improper behaviour when
creating and destroying xsk and umem structures.

Multiple calls to this function is documented as supported.

Fixes: ca7a83e2 ("libbpf: Only create rx and tx XDP rings when necessary")
Signed-off-by: Kev Jackson <foamdino@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/YL4aU4f3Aaik7CN0@linux-dev