perf PLT hit turns off symbol annotations

[David Miller]

PLT entries are anon memory on sparc once they are written to by the
dynamic linker.

But if we take a perf event within such a page, perf thinks it's this
JIT tmp/perf-***.map stuff.

Of course there won't be a real /tmp/perf-***.map file for this
anonymous memory, so dso__load() fails and this turns off symbol
annotation completely.

First of all, this is a very extreme way to handle being unable
to load a DSO.  We should just fail to annotate symbols in that
DSO, rather than everywhere.

Second of all, assuming code execution hits are in anonymous memory
must be JIT code is not correct either.  It can be in the PLT on
several architectures.
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