CVE-2026-53319: blk-wbt: remove WARN_ON_ONCE from wbt_init_enable_default()

In the Linux kernel, the following vulnerability has been resolved:

blk-wbt: remove WARNONONCE from wbtinitenabledefault()

wbtinitenabledefault() uses WARNONONCE to check for failures from wbtalloc() and wbtinit(). However, both are expected failure paths:

- wbtalloc() can return NULL under memory pressure (-ENOMEM) - wbtinit() can fail with -EBUSY if wbt is already registered

syzbot triggers this by injecting memory allocation failures during MTD partition creation via ioctl(BLKPG), causing a spurious warning.

wbtinitenabledefault() is a best-effort initialization called from blkregisterqueue() with a void return type. Failure simply means the disk operates without writeback throttling, which is harmless.

Replace WARNONONCE with plain if-checks, consistent with how wbtsetlat() in the same file already handles these failures. Add a prwarn() for the wbtinit() failure to retain diagnostic information without triggering a full stack trace.