libmount: (subdir) support detached open_tree() (>=6.15)

libmount: (subdir) support detached open_tree() (>=6.15)

The latest kernel can open a directory as a tree (open_tree()) on a
detached mount tree. This means we do not need to unshare and
attach the root of the filesystem to a private temporary directory. All
this machinery can be replaced by one open_tree() call.

Old version:
	fsopen("ext4", FSOPEN_CLOEXEC)          = 3
	unshare(CLONE_NEWNS)                    = 0
	fsconfig(3, FSCONFIG_SET_STRING, "source", "/dev/sdc", 0) = 0
	fsconfig(3, FSCONFIG_CMD_CREATE, NULL, NULL, 0) = 0
	fsmount(3, FSMOUNT_CLOEXEC, 0)          = 6
	move_mount(6, "", AT_FDCWD, "/run/mount/tmptgt", MOVE_MOUNT_F_EMPTY_PATH) = 0
	open_tree(6, "subdir", OPEN_TREE_CLONE|OPEN_TREE_CLOEXEC) = 7
	setns(4, CLONE_NEWNS)                   = 0
	move_mount(7, "", AT_FDCWD, "/mnt/test", MOVE_MOUNT_F_EMPTY_PATH) = 0
	setns(5, CLONE_NEWNS)                   = 0
	umount2("/run/mount/tmptgt", 0)         = 0
	setns(4, CLONE_NEWNS)                   = 0

New version:
	fsopen("ext4", FSOPEN_CLOEXEC)          = 3
	fsconfig(3, FSCONFIG_SET_STRING, "source", "/dev/sda", 0) = 0
	fsconfig(3, FSCONFIG_CMD_CREATE, NULL, NULL, 0) = 0
	fsmount(3, FSMOUNT_CLOEXEC, 0)          = 4
	open_tree(4, "subdir", OPEN_TREE_CLONE|OPEN_TREE_CLOEXEC|AT_SYMLINK_NOFOLLOW|AT_NO_AUTOMOUNT|AT_RECURSIVE
	move_mount(5, "", AT_FDCWD, "/mnt/test", MOVE_MOUNT_F_EMPTY_PATH) = 0

Note that this commit does not completely deactivate hook_subdir.c
because it is better to keep things in one place. It only adds a "subdir"
to the struct libmnt_sysapi to force hook_mount.c to call open_tree(subdir),
but all the logic and checks are still in hook_subdir.c.

Signed-off-by: Karel Zak <kzak@redhat.com>