perf: Use guard() for aux_mutex in perf_mmap()

perf: Use guard() for aux_mutex in perf_mmap()

After duplicating the common code into the rb/aux branches is it
possible to use a simple guard() for the aux_mutex. Making the aux
branch self-contained.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/r/20250812104019.246250452@infradead.org