alloc_tag: use str_on_off() helper

alloc_tag: use str_on_off() helper

Replace the ternary (enable ?  "on" : "off") with the str_on_off() helper
from string_choices.h.  This improves readability by replacing the
three-operand ternary with a single function call, ensures consistent
string output, and allows potential string deduplication by the linker,
resulting in a slightly smaller binary.

Link: https://lkml.kernel.org/r/20250814093827.237980-1-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>