netfilter: nft_set_hash: mark set element as dead when deleting from packet path

Set on the NFT_SET_ELEM_DEAD_BIT flag on this element, instead of
performing element removal which might race with an ongoing transaction.
Enable gc when dynamic flag is set on since dynset deletion requires
garbage collection after this patch.

Fixes: d0a8d877

 ("netfilter: nft_dynset: support for element deletion")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>