bpf: fix bpf_prog_array_copy_to_user() issues

1. move copy_to_user out of rcu section to fix the following issue:

./include/linux/rcupdate.h:302 Illegal context switch in RCU read-side critical section!
stack backtrace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 lockdep_rcu_suspicious+0x123/0x170 kernel/locking/lockdep.c:4592
 rcu_preempt_sleep_check include/linux/rcupdate.h:301 [inline]
 ___might_sleep+0x385/0x470 kernel/sched/core.c:6079
 __might_sleep+0x95/0x190 kernel/sched/core.c:6067
 __might_fault+0xab/0x1d0 mm/memory.c:4532
 _copy_to_user+0x2c/0xc0 lib/usercopy.c:25
 copy_to_user include/linux/uaccess.h:155 [inline]
 bpf_prog_array_copy_to_user+0x217/0x4d0 kernel/bpf/core.c:1587
 bpf_prog_array_copy_info+0x17b/0x1c0 kernel/bpf/core.c:1685
 perf_event_query_prog_array+0x196/0x280 kernel/trace/bpf_trace.c:877
 _perf_ioctl kernel/events/core.c:4737 [inline]
 perf_ioctl+0x3e1/0x1480 kernel/events/core.c:4757

2. move *prog under rcu, since it's not ok to dereference it afterwards

3. in a rare case of prog array being swapped between bpf_prog_array_length()
   and bpf_prog_array_copy_to_user() calls make sure to copy zeros to user space,
   so the user doesn't walk over uninited prog_ids while kernel reported
   uattr->query.prog_cnt > 0

Reported-by:  <syzbot+7dbcd2d3b85f9b608b23@syzkaller.appspotmail.com>
Fixes: 468e2f64

 ("bpf: introduce BPF_PROG_QUERY command")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>