perf symbol: Simplify kernel module checking

In dso__load(), it checks if the dso is a kernel module by looking the
symtab type. Actually dso has 'is_kmod' field to check that easily and
dso__set_module_info() set the symtab type and the is_kmod bit. So it
should have the same result to check the is_kmod bit.

Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240621170528.608772-3-namhyung@kernel.org