user_get_super() is now backed by the global device-to-superblock table
instead of a walk of the super_blocks list. ustat(2) is its most direct
user-visible consumer but nothing in the tree exercises it.
Add a small regression test: the device number of a mounted tmpfs (an
anonymous device, registered in the table by sget_fc()) must resolve,
it must stop resolving after the unmount (the entry is dropped again in
kill_super_notify()), and bogus device numbers keep reporting EINVAL.
The test passes on kernels before the conversion: it pins down the
semantics the table-backed lookup must preserve.
Link: https://patch.msgid.link/20260616-work-super-bdev_holder_global-v2-18-7df6b864028e@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
