net: kcm: Hold RCU read lock while running BPF parser

In the Linux kernel, the following vulnerability has been resolved: net: kcm: Hold RCU read lock while running BPF parser kcm_parse_func_strparser() calls bpf_prog_run_pin_on_cpu() which prevents CPU migration, but does not establish an RCU read-side critical section. Consequently, BPF map operations can trigger WARN_ON_ONCE(!bpf_rcu_lock_held()) when called from the KCM strparser program. Hold the RCU read lock while running the program.