drivers/base/memory: simplify outputting of valid_zones_show()

No need to specify position at the first writing to the buf because the
@len is always 0 at this time.  Use sysfs_emit() instead to simplify it. 
Also avoid setting/checking default_zone with a conditional operator.

Link: https://lkml.kernel.org/r/20250108015223.1522887-1-ruansy.fnst@fujitsu.com


Signed-off-by: Shiyang Ruan <ruansy.fnst@fujitsu.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>