bpf: Use kmalloc_nolock() universally in local storage

Switch to kmalloc_nolock() universally in local storage. Socket local
storage didn't move to kmalloc_nolock() when BPF memory allocator was
replaced by it for performance reasons. Now that kfree_rcu() supports
freeing memory allocated by kmalloc_nolock(), we can move the remaining
local storages to use kmalloc_nolock() and cleanup the cluttered free
paths.

Use kfree() instead of kfree_nolock() in bpf_selem_free_trace_rcu() and
bpf_local_storage_free_trace_rcu(). Both callbacks run in process context
where spinning is allowed, so kfree_nolock() is unnecessary.

Benchmark:

./bench -p 1 local-storage-create --storage-type socket \
  --batch-size {16,32,64}

The benchmark is a microbenchmark stress-testing how fast local storage
can be created. There is no measurable throughput change for socket local
storage after switching from kzalloc() to kmalloc_nolock().

Socket local storage

                 batch  creation speed              diff
---------------  ----   ------------------          ----
Baseline          16    433.9 ± 0.6 k/s
                  32    434.3 ± 1.4 k/s
                  64    434.2 ± 0.7 k/s

After             16    439.0 ± 1.9 k/s             +1.2%
                  32    437.3 ± 2.0 k/s             +0.7%
                  64    435.8 ± 2.5k/s              +0.4%

Also worth noting that the baseline got a 5% throughput boost when sheaf
replaces percpu partial slab recently [0].

[0] https://lore.kernel.org/bpf/20260123-sheaves-for-all-v4-0-041323d506f7@suse.cz/

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260411015419.114016-3-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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