fs: use all available ids

The counter is unconditionally incremented for each mount allocation.
If we set it to 1ULL << 32 we're losing 4294967296 as the first valid
non-32 bit mount id.

Link: https://lore.kernel.org/r/20240719-work-mount-namespace-v1-1-834113cab0d2@kernel.org


Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>