xdp: Fix race in dev_map_hash_update_elem() when replacing element

syzbot found a crash in dev_map_hash_update_elem(), when replacing an
element with a new one. Jesper correctly identified the cause of the crash
as a race condition between the initial lookup in the map (which is done
before taking the lock), and the removal of the old element.

Rather than just add a second lookup into the hashmap after taking the
lock, fix this by reworking the function logic to take the lock before the
initial lookup.

Fixes: 6f9d451a ("xdp: Add devmap_hash map type for looking up devices by hashed index")
Reported-and-tested-by:  <syzbot+4e7a85b1432052e8d6f8@syzkaller.appspotmail.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>