fuse: add FOPEN_NOFLUSH

Add flag returned by FUSE_OPEN and FUSE_CREATE requests to avoid flushing
data cache on close.

Different filesystems implement ->flush() is different ways:
 - Most disk filesystems do not implement ->flush() at all
 - Some network filesystem (e.g. nfs) flush local write cache of
   FMODE_WRITE file and send a "flush" command to server
 - Some network filesystem (e.g. cifs) flush local write cache of
   FMODE_WRITE file without sending an additional command to server

FUSE flushes local write cache of ANY file, even non FMODE_WRITE
and sends a "flush" command to server (if server implements it).

The FUSE implementation of ->flush() seems over agressive and
arbitrary and does not make a lot of sense when writeback caching is
disabled.

Instead of deciding on another arbitrary implementation that makes
sense, leave the choice of per-file flush behavior in the hands of
the server.

Link: https://lore.kernel.org/linux-fsdevel/CAJfpegspE8e6aKd47uZtSYX8Y-1e1FWS0VL0DH2Skb9gQP5RJQ@mail.gmail.com/


Suggested-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>