loop: fix partition scan race between udev and loop_reread_partitions()

When LOOP_CONFIGURE is called with LO_FLAGS_PARTSCAN, the following
sequence occurs:

1. disk_force_media_change() sets GD_NEED_PART_SCAN
2. Uevent suppression is lifted and a KOBJ_CHANGE uevent is sent
3. loop_global_unlock() releases the lock
4. loop_reread_partitions() calls bdev_disk_changed() to scan

There is a race between steps 2 and 4: when udev receives the uevent
and opens the device before loop_reread_partitions() runs,
blkdev_get_whole() in bdev.c sees GD_NEED_PART_SCAN set and calls
bdev_disk_changed() for a first scan. Then loop_reread_partitions()
does a second scan. The open_mutex serializes these two scans, but
does not prevent both from running.

The second scan in bdev_disk_changed() drops all partition devices
from the first scan (via blk_drop_partitions()) before re-adding
them, causing partition block devices to briefly disappear. This
breaks any systemd unit with BindsTo= on the partition device: systemd
observes the device going dead, fails the dependent units, and does
not retry them when the device reappears.

Fix this by removing the GD_NEED_PART_SCAN set from
disk_force_media_change() entirely. None of the current callers need
the lazy on-open partition scan triggered by this flag:

- floppy: sets GENHD_FL_NO_PART, so disk_has_partscan() is always
false and GD_NEED_PART_SCAN has no effect.
- loop (loop_configure, loop_change_fd): when LO_FLAGS_PARTSCAN is
set, loop_reread_partitions() performs an explicit scan. When not
set, GD_SUPPRESS_PART_SCAN prevents the lazy scan path.
- loop (__loop_clr_fd): calls bdev_disk_changed() explicitly if
LO_FLAGS_PARTSCAN is set.
- nbd (nbd_clear_sock_ioctl): capacity is set to zero immediately
after; nbd manages GD_NEED_PART_SCAN explicitly elsewhere.

With GD_NEED_PART_SCAN no longer set by disk_force_media_change(),
udev opening the loop device after the uevent no longer triggers a
redundant scan in blkdev_get_whole(), and only the single explicit
scan from loop_reread_partitions() runs.

A regression test for this bug has been submitted to blktests:
https://github.com/linux-blktests/blktests/pull/240.

Fixes: 9f65c489b68d ("loop: raise media_change event")
Signed-off-by: Daan De Meyer <daan@amutable.com>
Acked-by: Christian Brauner <brauner@kernel.org>
Link: https://patch.msgid.link/20260331105130.1077599-1-daan@amutable.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>