fuse: allow O_PATH fd for FUSE_DEV_IOC_BACKING_OPEN

Only f_path is used from backing files registered with
FUSE_DEV_IOC_BACKING_OPEN, so it makes sense to allow O_PATH descriptors.

O_PATH files have an empty f_op, so don't check read_iter/write_iter.

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