kernel: bpf: Fix a race condition between btf_put() and map_free()

value_rec' is a pointer to the record in struct_metas_tab.
And it is possible that that particular record has been freed by
btf_struct_metas_free() and hence we have a kasan error here.

v1 of the patch ([2]) moves btf_put() after map_free callback as a suggested fix.