nfs: flag as supporting FOP_DONTCACHE

nfs: flag as supporting FOP_DONTCACHE

Jens says: "nfs just uses generic_file_read_iter(), so read side is
fine with the flag added and generic_perform_write() for the write
side, wrapped in some nfs jazz. So you can probably just set the flag
and be done with it."

Must also update nfs_write_begin() to set FGP_DONTCACHE in fgp flags
passed to __filemap_get_folio().

Suggested-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>