net/sched: ematch: check nla_put_nohdr() return value in tcf_em_tree_dump()

When tcf_em_tree_dump() was converted from the legacy rtnetlink
`RTA_PUT_NOHDR` macro (which contained an implicit `goto
rtattr_failure;`) to `nla_put_nohdr()`, the return value check on
`nla_put_nohdr()` was omitted for container/simple matches and raw match
payloads.

If the netlink dump skb runs out of tailroom at `nla_put_nohdr()`, the
error is silently ignored and `match_start->nla_len` is finalized
without the match payload, emitting a malformed/truncated
TCA_EMATCH_TREE_LIST attribute to userspace instead of returning `-1`
via `nla_put_failure` so rtnetlink can reallocate and retry the dump.

Check the return value of `nla_put_nohdr()` in both branches and jump to
`nla_put_failure` on error.

Cc: stable+noautosel@kernel.org # never worked
Signed-off-by: Hui Peng <benquike@gmail.com>
Link: https://patch.msgid.link/20260919210624.3030344-1-benquike@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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