fs: actually hold the namespace semaphore

Don't use a scoped guard that only protects the next statement.

Use a regular guard to make sure that the namespace semaphore is held
across the whole function.

Signed-off-by: Christian Brauner <brauner@kernel.org>
Reported-by: Leon Romanovsky <leon@kernel.org>
Link: https://lore.kernel.org/all/20250401170715.GA112019@unreal/


Fixes: db04662e ("fs: allow detached mounts in clone_private_mount()")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>