fs: use a consume fence in mnt_idmap()

The routine is used in link_path_walk() for every path component.

To my reading the entire point of the fence was to grab a fully
populated mnt_idmap, but that's already going to happen with mere
consume fence.

Eliminates an actual fence on arm64.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20241130051712.1036527-1-mjguzik@gmail.com


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