fuse: Switch to fc_mount() for submounts

fc_mount() already handles the vfs_get_tree(), sb->s_umount
unlocking and vfs_create_mount() sequence. Using it greatly
simplifies fuse_dentry_automount().

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>