Bug 2393487 (CVE-2025-39686)

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

comedi: Make insn_rw_emulate_bits() do insn->n samples

The `insn_rw_emulate_bits()` function is used as a default handler for
`INSN_READ` instructions for subdevices that have a handler for
`INSN_BITS` but not for `INSN_READ`.  Similarly, it is used as a default
handler for `INSN_WRITE` instructions for subdevices that have a handler
for `INSN_BITS` but not for `INSN_WRITE`. It works by emulating the
`INSN_READ` or `INSN_WRITE` instruction handling with a constructed
`INSN_BITS` instruction.  However, `INSN_READ` and `INSN_WRITE`
instructions are supposed to be able read or write multiple samples,
indicated by the `insn->n` value, but `insn_rw_emulate_bits()` currently
only handles a single sample.  For `INSN_READ`, the comedi core will
copy `insn->n` samples back to user-space.  (That triggered KASAN
kernel-infoleak errors when `insn->n` was greater than 1, but that is
being fixed more generally elsewhere in the comedi core.)

Make `insn_rw_emulate_bits()` either handle `insn->n` samples, or return
an error, to conform to the general expectation for `INSN_READ` and
`INSN_WRITE` handlers.