fs: look up the superblock via the device table in user_get_super()

user_get_super() still finds the superblock for a device number by
walking the global super_blocks list under sb_lock. Every superblock is
registered in the device table under its s_dev since sget_fc() inserts
it there, including superblocks on anonymous devices, so use the table
instead.

The refcount-pinning cursor helpers super_dev_{get,first,next}() only
touch table state and do not depend on CONFIG_BLOCK, so drop the
CONFIG_BLOCK guard around them: their new caller serves anonymous
devices as well (ustat() on e.g. tmpfs) and is built without
CONFIG_BLOCK. The guard falls in this patch rather than separately
since without this caller the helpers would be unused without
CONFIG_BLOCK.

The pinned entry holds a passive reference on the superblock so
super_lock() can be called directly; once the superblock is locked grab
a passive reference for the caller before dropping the pin.

The device table contains more than the old walk could find: a
superblock is also registered for every additional device it claims
(the xfs log and realtime devices, btrfs member devices, the ext4
external journal, erofs blob devices). Don't filter those out:
specifying any device a filesystem uses now resolves to that
filesystem, so ustat() and quotactl() work on e.g. the xfs log device
or a btrfs member device (the latter used to fail outright as btrfs
superblocks carry an anonymous s_dev that never matches a member
device). When several superblocks share a device (erofs blob devices)
the first live superblock wins.

The cursor also keeps scanning past dying superblocks where the old
walk gave up after the first s_dev match, so a mount racing with the
unmount of the same device (or with the reuse of a recycled anonymous
dev_t) finds the live superblock where the old walk could spuriously
return NULL.

This removes the last s_dev-keyed walk of the super_blocks list and
takes ustat() and quotactl()'s block device lookup off sb_lock
entirely.

Link: https://patch.msgid.link/20260616-work-super-bdev_holder_global-v2-17-7df6b864028e@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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