blk-wbt: remove WARN_ON_ONCE from wbt_init_enable_default()

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

blk-wbt: remove WARN_ON_ONCE from wbt_init_enable_default()

wbt_init_enable_default() uses WARN_ON_ONCE to check for failures from

wbt_alloc() and wbt_init(). However, both are expected failure paths:

- wbt_alloc() can return NULL under memory pressure (-ENOMEM)

- wbt_init() 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.

wbt_init_enable_default() is a best-effort initialization called from

blk_register_queue() with a void return type. Failure simply means the

disk operates without writeback throttling, which is harmless.

Replace WARN_ON_ONCE with plain if-checks, consistent with how

wbt_set_lat() in the same file already handles these failures. Add a

pr_warn() for the wbt_init() failure to retain diagnostic information

without triggering a full stack trace.