perf/core: Exit early on perf_mmap() fail

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

perf/core: Exit early on perf_mmap() fail

When perf_mmap() fails to allocate a buffer, it still invokes the

event_mapped() callback of the related event. On X86 this might increase

the perf_rdpmc_allowed reference counter. But nothing undoes this as

perf_mmap_close() is never called in this case, which causes another

reference count leak.

Return early on failure to prevent that.