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