fs: maintain a global device-to-superblock table

fs_holder_ops recovers the owning superblock from bdev->bd_holder, which
forces the holder to be exactly one superblock and prevents several
superblocks from sharing one block device. That's what erofs is doing.

As a first step introduce a global dev_t-keyed rhltable mapping each
device to the superblock(s) using it. The entry is preallocated in
alloc_super() and registered under sb->s_dev by the set callback through
set_anon_super() and set_bdev_super(), the two helpers every set
callback assigns s_dev through. Registration is the final fallible act
of a set callback, so an insert failure unwinds through sget_fc()'s
existing set-failure path: the fs_context keeps ownership of s_fs_info
and the callers' error paths stay correct. set_anon_super() releases
the anonymous dev it allocated when registration fails. Unwinding
through deactivate_locked_super() instead would run kill_sb() and free
s_fs_info behind the caller's back: nfs and ceph free that object
through a local pointer when sget_fc() fails and would double-free.

The superblock stashes the entry in sb->s_super_dev and
kill_super_notify() drops the claim through it, so teardown doesn't
depend on s_dev staying stable; an entry that was never registered is
freed together with the superblock in destroy_super_work().

Each table entry holds a passive reference (s_passive) on its
superblock, so the struct stays valid for as long as the entry is
reachable. Entries are claim-counted through sd_ref: additional claims
on the same (device, superblock) pair share the entry, and the unlink
is deferred to the last put, so a later iteration cursor never resumes
from a removed node.

The table is initialized from mnt_init(): the first superblocks (the
tmpfs shm mount and rootfs) are created from start_kernel() long before
any initcall runs, so an initcall would be too late.

The table has no readers yet; the fs_holder_ops callbacks are switched
over once all devices a filesystem claims are registered.

Link: https://patch.msgid.link/20260616-work-super-bdev_holder_global-v2-7-7df6b864028e@kernel.org
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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