CVE-2026-89770: iomap: don't free integrity payload that doesn't exist

In the Linux kernel, the following vulnerability has been resolved: iomap: don't free integrity payload that doesn't exist fs_bio_integrity_alloc might not allocate a bio integrity payload if PI verification is disabled on the block device. Check for that case before calling fs_bio_integrity_free in iomap_bio_read_folio_range_sync to avoid a NULL pointer dereferences. Make the branch cover the PI verification as well - while fs_bio_integrity_verify works without an integrity payload, it requires one to actually do useful work.