perf/core: Handle buffer mapping fail correctly in perf_mmap()

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

perf/core: Handle buffer mapping fail correctly in perf_mmap()

After successful allocation of a buffer or a successful attachment to an
existing buffer perf_mmap() tries to map the buffer read only into the page
table. If that fails, the already set up page table entries are zapped, but
the other perf specific side effects of that failure are not handled. The
calling code just cleans up the VMA and does not invoke perf_mmap_close().

This leaks reference counts, corrupts user->vm accounting and also results
in an unbalanced invocation of event::event_mapped().

Cure this by moving the event::event_mapped() invocation before the
map_range() call so that on map_range() failure perf_mmap_close() can be
invoked without causing an unbalanced event::event_unmapped() call.

perf_mmap_close() undoes the reference counts and eventually frees buffers.