crimson/common: skip first 4 frames when dumping a backtrace.

crimson/common: skip first 4 frames when dumping a backtrace.

It's all about these items:

```
 0# print_backtrace(std::basic_string_view<char, std::char_traits<char> >) at /home/rzarzynski/ceph1/build/../src/crimson/common/fatal_signal.cc:80
 1# FatalSignal::signaled(int, siginfo_t const&) at /opt/rh/gcc-toolset-9/root/usr/include/c++/9/ostream:570
 2# FatalSignal::install_oneshot_signal_handler<11>()::{lambda(int, siginfo_t*, void*)#1}::_FUN(int, siginfo_t*, void*) at /home/rzarzynski/ceph1/build/../src/crimson/common/fatal_signal.cc:
62
 3# 0x00007F16BBA13B30 in /lib64/libpthread.so.0
```

They are part of our backtrace handling and typically developers
are not interested in them. Let's be consistent with the classical
OSD and hide them.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>