iomap: make buffered writes work with RWF_DONTCACHE

iomap: make buffered writes work with RWF_DONTCACHE

Add iomap buffered write support for RWF_DONTCACHE. If RWF_DONTCACHE is
set for a write, mark the folios being written as uncached. Then
writeback completion will drop the pages. The write_iter handler simply
kicks off writeback for the pages, and writeback completion will take
care of the rest.

This still needs the user of the iomap buffered write helpers to call
folio_end_dropbehind() upon successful issue of the writes.

Signed-off-by: Jens Axboe <axboe@kernel.dk>