block: initialize integrity buffer to zero before writing it to media

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

block: initialize integrity buffer to zero before writing it to media

Metadata added by bio_integrity_prep is using plain kmalloc, which leads

to random kernel memory being written media. For PI metadata this is

limited to the app tag that isn't used by kernel generated metadata,

but for non-PI metadata the entire buffer leaks kernel memory.

Fix this by adding the __GFP_ZERO flag to allocations for writes.