Use kdump_vmcoreinfo_symbol in use_kernel_utsname()

Now that there is a convenience function, let's use it internally.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Cleanup: Separate function for new_utsname in init_uts_ns

This code makes use_kernel_utsname() much harder to understand. Let's
move it to a separate function.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Use SYMBOL(system_utsname) if available

Pre-2.6.19 kernel did not have init_uts_ns, but they exported the
system_utsname symbol instead. Use it if it exists.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>