fs: put the old fs_struct before the old namespaces in unshare()

unshare(CLONE_NEWNS) always copies the fs_struct and copy_mnt_ns()
points root and pwd of that copy into the new mount namespace. The old
fs_struct pins the old root and pwd until ksys_unshare() frees it at the
end and switch_task_namespaces() will already put the old mount
namespace before.

This causes pointless work in user namespaces because the copied mounts
are locked and stay connected. Switch and free the fs_struct before
switching the namespaces. The old root and pwd are wasted while their
mount namespace is still alive which also keeps both puts on the
mntput() fast path.

setns() already does this right as it updates root and pwd before the
namespaces are switched.

Link: https://patch.msgid.link/20260926-work-mount-fixes-2-v1-7-f357abf3d17b@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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