comedi: remove the mapping of the Comedi buffer in vmalloc address space

comedi: remove the mapping of the Comedi buffer in vmalloc address space

Now that all the code that accesses the Comedi buffer data does so
page-by-page, using the `virt_addr` member of `struct comedi_buf_page`
to point to the data of each page, do not linearly map the buffer into
vmalloc address space (pointed to by the `prealloc_buf` member of
`struct comedi_async`).  That was only done for convenience, but was not
done for those drivers that need a DMA coherent buffer, which is
allocated in a single chunk.  Remove the `prealloc_buf` member as it is
no longer used.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20250415114008.5977-4-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>