fuse: fix parallel dio write on file open in passthrough mode

Parallel dio write takes a negative refcount of fi->iocachectr and so does
open of file in passthrough mode.

The refcount of passthrough mode is associated with attach/detach of a
fuse_backing object to fuse inode.

For parallel dio write, the backing file is irrelevant, so the call to
fuse_inode_uncached_io_start() passes a NULL fuse_backing object.

Passing a NULL fuse_backing will result in false -EBUSY error if the file
is already open in passthrough mode.

Allow taking negative fi->iocachectr refcount with NULL fuse_backing,
because it does not conflict with an already attached fuse_backing object.

Fixes: 4a90451b

 ("fuse: implement open in passthrough mode")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>