bpf: Protect the struct_ops association with a spinlock

bpf_prog_assoc_struct_ops() and bpf_prog_disassoc_struct_ops() update
prog->aux->st_ops_assoc under st_ops_assoc_mutex, and the latter puts
the map of a non-struct_ops prog while holding it.

A struct_ops map is about to dissociate its progs from
map_free_pre_rcu(), which bpf_map_put() calls in whatever context drops
the last reference, including softirq, e.g. bpf_struct_ops_put() from
tcp_cleanup_congestion_control(). A mutex can't be taken there, and the
put of the last reference under the lock of one prog would take the
lock of other progs.

Replace the mutex with a spinlock taken with interrupts disabled, and
put the map after dropping it.

Signed-off-by: Qiliang Yuan <odys.yuan@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261022-selftests-bpf-struct-ops-assoc-timer-v6-1-2c0f18e82582@gmail.com

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