perf: aux buffer and mmaping PROT_WRITE

[Vince Weaver]

So I came across this issue when improving mmap() support in the 
perf_fuzzer.

For AUX buffers, you can specify PROT_WRITE when allocating the buffer 
using mmap(), and it will return a buffer just fine, but if you actually 
try to write anything you get an instant SIGBUS.

Is this expected behavior?  It seems like we should fail the initial 
allocation if we ask for a writable buffer but it is not writable.

Vince
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