xdp: obtain the mem_id mutex before trying to remove an entry.

A lockdep splat was observed when trying to remove an xdp memory
model from the table since the mutex was obtained when trying to
remove the entry, but not before the table walk started:

Fix the splat by obtaining the lock before starting the table walk.

Fixes: c3f812ce ("page_pool: do not release pool until inflight == 0.")
Reported-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Tested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>