bpf: Add missing annotations for __bpf_prog_enter() and __bpf_prog_exit()

Sparse reports a warning at __bpf_prog_enter() and __bpf_prog_exit()

warning: context imbalance in __bpf_prog_enter() - wrong count at exit
warning: context imbalance in __bpf_prog_exit() - unexpected unlock

The root cause is the missing annotation at __bpf_prog_enter()
and __bpf_prog_exit()

Add the missing __acquires(RCU) annotation
Add the missing __releases(RCU) annotation
Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200311010908.42366-2-jbi.octave@gmail.com

kernel/bpf/trampoline.c

@@ -352,6 +352,7 @@ void bpf_trampoline_put(struct bpf_trampoline *tr)
  * call __bpf_prog_exit
  */
 u64 notrace __bpf_prog_enter(void)
+	__acquires(RCU)
 {
 	u64 start = 0;
 
@@ -363,6 +364,7 @@ u64 notrace __bpf_prog_enter(void)
 }
 
 void notrace __bpf_prog_exit(struct bpf_prog *prog, u64 start)
+	__releases(RCU)
 {
 	struct bpf_prog_stats *stats;