mm: add bdi_set_max_bytes() function

This introduces the bdi_set_max_bytes() function. The max_bytes function
does not store the max_bytes value. Instead it converts the max_bytes
value into the corresponding ratio value.

Link: https://lkml.kernel.org/r/20221119005215.3052436-8-shr@devkernel.io


Signed-off-by: Stefan Roesch <shr@devkernel.io>
Cc: Chris Mason <clm@meta.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>