btrfs: make sure we rescan all devices after unregistering

btrfs: make sure we rescan all devices after unregistering

There are some btrfs test cases utilizing
_btrfs_forget_or_module_reload() to unregister all btrfs devices.

However _btrfs_forget_or_module_reload() will unregister all devices,
meaning if TEST_DEV is part of a multi-device btrfs, after those test
cases TEST_DEV will no longer be mountable.

This patch will introduce a new function, btrfs_rescan_devices() to undo
the unregister, so that all later test cases can mount TEST_DEV without
any problem.

Since we are here, also add a missing
_require_btrfs_forget_or_module_loadable for btrfs/225.

Reported-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>