comedi: access buffer data page-by-page

The aim is to get rid of the `prealloc_buf` member of `struct
comedi_async` and access the buffer contents on a page-by-page basis
using the addresses in the `virt_addr` member of `struct
comedi_buf_page`.  This will allow us to eliminate a `vmap()` that maps
the whole buffer.

Since the buffer pages have non-consecutive `virt_addr` addresses in
virtual memory (except for drivers using DMA), change the loops that
access buffer data to access it page-by-page.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20250415114008.5977-3-abbotti@mev.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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