bpf, offload: Unlock on error in bpf_offload_dev_create()

We need to drop the bpf_devs_lock on error before returning.

Fixes: 9fd7c555 ("bpf: offload: aggregate offloads per-device")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Link: https://lore.kernel.org/bpf/20191104091536.GB31509@mwanda