fs: record sequence number of origin mount namespace

Store the sequence number of the mount namespace the anonymous mount
namespace has been created from. This information will be used in
follow-up patches.

Link: https://lore.kernel.org/r/20250221-brauner-open_tree-v1-1-dbcfcb98c676@kernel.org


Signed-off-by: Christian Brauner <brauner@kernel.org>