netfilter: nf_tables_offload: suppress WARN_ON_ONCE for ENOMEM in abort path

In the Linux kernel, the following vulnerability has been resolved:

netfilter: nf_tables_offload: suppress WARN_ON_ONCE for ENOMEM in abort path

In nft_flow_rule_offload_abort(), WARN_ON_ONCE(err) is triggered on every
error during rollback, including -ENOMEM. Memory allocation failures are
expected under low-memory conditions and do not indicate a kernel bug.

Trace for example:
nft_flow_offload_chain() // FLOW_BLOCK_BIND
nft_flow_block_chain()
nft_chain_offload_cmd()
nft_block_offload_cmd()
->ndo_setup_tc()
nsim_setup_tc()
flow_block_cb_setup_simple()
flow_block_cb_alloc() // fails to -ENOMEM

The warning was reproduced on the 5.10 stable kernel under memory pressure
via fault injection, but the underlying bug exists in mainline as well,
as demonstrated by the ENOMEM trace above. The following splat was
triggered during nf_tables transaction processing:

WARNING: CPU: 0 PID: 8567 at net/netfilter/nf_tables_offload.c:532 nft_flow_rule_offload_abort net/netfilter/nf_tables_offload.c:532 [inline]
WARNING: CPU: 0 PID: 8567 at net/netfilter/nf_tables_offload.c:532 nft_flow_rule_offload_commit+0x971/0xcd0 net/netfilter/nf_tables_offload.c:591
Modules linked in:
CPU: 0 PID: 8567 Comm: syz-executor.0 Not tainted 5.10.260-syzkaller #0
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
RIP: 0010:nft_flow_rule_offload_abort net/netfilter/nf_tables_offload.c:532 [inline]
RIP: 0010:nft_flow_rule_offload_commit+0x971/0xcd0 net/netfilter/nf_tables_offload.c:591
Call Trace:
nf_tables_commit+0x3bd/0x4bd0 net/netfilter/nf_tables_api.c:8604
nfnetlink_rcv_batch+0xb1e/0x1f20 net/netfilter/nfnetlink.c:509
nfnetlink_rcv_skb_batch net/netfilter/nfnetlink.c:579 [inline]
nfnetlink_rcv+0x3b3/0x420 net/netfilter/nfnetlink.c:597
netlink_unicast_kernel net/netlink/af_netlink.c:1314 [inline]
netlink_unicast+0x6cd/0xa00 net/netfilter/af_netlink.c:1340
netlink_sendmsg+0x906/0xe10 net/netfilter/af_netlink.c:1919
sock_sendmsg_nosec net/socket.c:651 [inline]
__sock_sendmsg+0x155/0x190 net/socket.c:663
____sys_sendmsg+0x705/0x870 net/socket.c:2379
___sys_sendmsg+0x100/0x170 net/socket.c:2433
__sys_sendmsg+0xe9/0x1c0 net/socket.c:2462
do_syscall_64+0x33/0x40 arch/x86/entry/common.c:46
entry_SYSCALL_64_after_hwframe+0x67/0xd1

Change the condition to WARN_ON_ONCE(err && err != -ENOMEM) so that
warnings are only emitted for unexpected errors. This aligns with the
common kernel practice of not warning on -ENOMEM.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.