btrfs: lift uuid_mutex to callers of btrfs_open_devices

btrfs: lift uuid_mutex to callers of btrfs_open_devices

[ Upstream commit f5194e34cabaddd348a90f950e0a8188dd26cdc0 ]

Prepartory work to fix race between mount and device scan.

The callers will have to manage the critical section, eg. mount wants to
scan and then call btrfs_open_devices without the ioctl scan walking in
and modifying the fs devices in the meantime.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>