perf/core: Preserve AUX buffer allocation failure result

A recent overhaul sets the return value to 0 unconditionally after the
allocations, which causes reference count leaks and corrupts the user->vm
accounting.

Preserve the AUX buffer allocation failure return value, so that the
subsequent code works correctly.

Fixes: 0983593f ("perf/core: Lift event->mmap_mutex in perf_mmap()")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: stable@vger.kernel.org