bpf: Fix bpf_sk_select_reuseport() memory leak

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix bpf_sk_select_reuseport() memory leak

As pointed out in the original comment, lookup in sockmap can return a TCP

ESTABLISHED socket. Such TCP socket may have had SO_ATTACH_REUSEPORT_EBPF

set before it was ESTABLISHED. In other words, a non-NULL sk_reuseport_cb

does not imply a non-refcounted socket.

Drop sk's reference in both error paths.

unreferenced object 0xffff888101911800 (size 2048):

comm "test_progs", pid 44109, jiffies 4297131437

hex dump (first 32 bytes):

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

80 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

backtrace (crc 9336483b):

__kmalloc_noprof+0x3bf/0x560

__reuseport_alloc+0x1d/0x40

reuseport_alloc+0xca/0x150

reuseport_attach_prog+0x87/0x140

sk_reuseport_attach_bpf+0xc8/0x100

sk_setsockopt+0x1181/0x1990

do_sock_setsockopt+0x12b/0x160

__sys_setsockopt+0x7b/0xc0

__x64_sys_setsockopt+0x1b/0x30

do_syscall_64+0x93/0x180

entry_SYSCALL_64_after_hwframe+0x76/0x7e