netfilter: nf_tables: bump set->ndeact on set flush

Add missing set->ndeact update on each deactivated element from the set
flush path. Otherwise, sets with fixed size break after flush since
accounting breaks.

 # nft add set x y { type ipv4_addr\; size 2\; }
 # nft add element x y { 1.1.1.1 }
 # nft add element x y { 1.1.1.2 }
 # nft flush set x y
 # nft add element x y { 1.1.1.1 }
 <cmdline>:1:1-28: Error: Could not process rule: Too many open files in system

Fixes: 8411b644 ("netfilter: nf_tables: support for set flushing")
Reported-by: Elise Lennion <elise.lennion@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>