net: bpf: correctly handle errors in sk_attach_filter()

Commit "net: bpf: make eBPF interpreter images read-only" has changed bpf_prog
to be vmalloc()ed but never handled some of the errors paths of the old code.

On error within sk_attach_filter (which userspace can easily trigger), we'd
kfree() the vmalloc()ed memory, and leak the internal bpf_work_struct.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>