block: fix memory leak in __blkdev_issue_zero_pages

In the Linux kernel, the following vulnerability has been resolved: block: fix memory leak in __blkdev_issue_zero_pages Move the fatal signal check before bio_alloc() to prevent a memory leak when BLKDEV_ZERO_KILLABLE is set and a fatal signal is pending. Previously, the bio was allocated before checking for a fatal signal. If a signal was pending, the code would break out of the loop without freeing or chaining the just-allocated bio, causing a memory leak. This matches the pattern already used in __blkdev_issue_write_zeroes() where the signal check precedes the allocation.