perf: Fix refcount warning on event->mmap_count increment

In the Linux kernel, the following vulnerability has been resolved:

perf: Fix refcount warning on event->mmap_count increment

When calling refcount_inc(&event->mmap_count) inside perf_mmap_rb(), the
following warning is triggered:

refcount_t: addition on 0; use-after-free.
WARNING: lib/refcount.c:25

PoC:

struct perf_event_attr attr = {0};
int fd = syscall(__NR_perf_event_open, &attr, 0, -1, -1, 0);
mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
int victim = syscall(__NR_perf_event_open, &attr, 0, -1, fd,
PERF_FLAG_FD_OUTPUT);
mmap(NULL, 0x3000, PROT_READ | PROT_WRITE, MAP_SHARED, victim, 0);

This occurs when creating a group member event with the flag
PERF_FLAG_FD_OUTPUT. The group leader should be mmap-ed and then mmap-ing
the event triggers the warning.

Since the event has copied the output_event in perf_event_set_output(),
event->rb is set. As a result, perf_mmap_rb() calls
refcount_inc(&event->mmap_count) when event->mmap_count = 0.

Disallow the case when event->mmap_count = 0. This also prevents two
events from updating the same user_page.