fuse: add submount support to <uapi/linux/fuse.h>

- Add fuse_attr.flags

- Add FUSE_ATTR_SUBMOUNT

  This is a flag for fuse_attr.flags that indicates that the given entry
  resides on a different filesystem than the parent, and as such should
  have a different st_dev.

- Add FUSE_SUBMOUNTS

  The client sets this flag if it supports automounting directories.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>