iov_iter: fix NULL deref in iov_iter_alignment()

Since commit 111a12b422a8 ("io_uring/rsrc: reject zero-length fixed
buffer import") io_import_fixed() hands out an empty ITER_BVEC for a
zero-length request. Its count is 0 and its bvec array is NULL.

iov_iter_alignment() only checks the count for ITER_UBUF. For ITER_BVEC
it calls iov_iter_alignment_bvec() and the do-while loop in there reads
bvec->bv_len before it looks at the count. So it dereferences NULL.

btrfs checks the alignment of a direct read with iov_iter_alignment()
before it looks at the count and f2fs does the same. So any user that
can open a file on btrfs can oops the kernel with a single zero-length
IORING_OP_READ_FIXED on an O_DIRECT file descriptor. With IOSQE_ASYNC
the oops happens in an io-wq worker. The dead worker stays on the worker
list and when the task that submitted the request exits io-wq writes to
the freed task_struct of the worker. That's just fallout from the oops
though.

Empty iterators with a NULL array aren't unusual. afs, tls and tipc
create them as well. So leave io_uring alone and make
iov_iter_alignment() return 0 for an empty iterator of any type.

Fixes: 111a12b422a8 ("io_uring/rsrc: reject zero-length fixed buffer import")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Link: https://patch.msgid.link/20261029-work-fs-ai-fixes-v1-3-88f22792f6f7@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>