kgdbts: Switch to do_sys_openat2() for breakpoint testing

Currently kgdbts can get stuck waiting for do_sys_open() to be called
in some of the current tests. This is because C compilers often
automatically inline this function, which is a very thin wrapper around
do_sys_openat2(), into some of its callers. gcc-10 does this on (at least)
both x86 and arm64.

We can fix the test suite by placing the breakpoints on do_sys_openat2()
instead since that isn't (currently) inlined. However do_sys_openat2() is
a static function so we cannot simply use an addressof. Since we are
testing debug machinery it is acceptable to use kallsyms to lookup a
suitable address because this is more or less what kdb does in the same
circumstances. Re-implement lookup_addr() to be based on kallsyms rather
than function pointers.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20210325094807.3546702-1-daniel.thompson@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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