kdumpctl: Fix return value of get_dump_mode_by_kernel

get_dump_mode_by_kernel always be called in a subshell, "exit" doesn't
make sense in this case, use return instead.

And we should return a non-zero value if there are any error occurs.

Signed-off-by: Lichen Liu <[email protected]>

Author: licliu

kdumpctl

@@ -1500,7 +1500,7 @@ get_dump_mode_by_kernel()
 		echo -n "$_dump_mode"
 	else
 		derror "failed to get dump mode for kernel $_kernel_path"
-		exit
+		return 1
 	fi
 }