printk: Remove unnecessary kmalloc() from syslog during clear

When the request is only for clearing logs, there is no need for
allocation/deallocation. Only the indexes need to be reset and returned.
Rest of the patch is mostly made up of changes because of indention.

Link: http://lkml.kernel.org/r/20180620135951epcas5p3bd2a8f25ec689ca333bce861b527dba2~54wyKcT0_3155531555epcas5p3y@epcas5p3.samsung.com
Cc: linux-kernel@vger.kernel.org
Cc: pankaj.m@samsung.com
Cc: a.sahrawat@samsung.com
Signed-off-by: Namit Gupta <gupta.namit@samsung.com>
Signed-off-by: Himanshu Maithani <himanshu.m@samsung.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>