block: allow making a block device unfreezable

Add bdev_deny_freeze() and bdev_allow_freeze(), modeled on
deny_write_access()/allow_write_access().  bd_fsfreeze_count becomes a
signed counter: > 0 counts active freezes, < 0 counts deniers, and the
two regimes are mutually exclusive.  bdev_freeze() refuses with -EBUSY
while a deny is held, and bdev_deny_freeze() refuses while the device is
frozen.

A filesystem that mutates a device's membership (a btrfs device add,
remove or replace) denies freezing on the device for the duration, so a
claim a freeze walk might act on is never added or torn down behind the
freezer's back.

The deny/allow helpers are a single atomic on bd_fsfreeze_count and take
no lock, so they can be called while holding s_umount without inverting
against bdev_freeze()'s bd_fsfreeze_mutex -> s_umount order.

Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20260616-work-super-freeze_deny_upstream-v2-1-b3567c7f994b@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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