fuse: remove useless IOCB_DIRECT in fuse_direct_read/write_iter

Commit 23c94e1c ("fuse: Switch to using async direct IO
for FOPEN_DIRECT_IO") gave the async direct IO code path in the
fuse_direct_read_iter() and fuse_direct_write_iter(). But since
these two functions are only called under FOPEN_DIRECT_IO is set,
it seems that we can also use the async direct IO even the flag
IOCB_DIRECT is not set to enjoy the async direct IO method. Also
move the definition of fuse_io_priv to where it is used in fuse_
direct_write_iter.

Signed-off-by: yangyun <yangyun50@huawei.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>