exfat: fix the new buffer was not zeroed before writing

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

exfat: fix the new buffer was not zeroed before writing

Before writing, if a buffer_head marked as new, its data must

be zeroed, otherwise uninitialized data in the page cache will

be written.

So this commit uses folio_zero_new_buffers() to zero the new

buffers before ->write_end().