btrfs: don't force mounts to wait for cleaner_kthread to delete one or more subvolumes

During a mount, we start the cleaner kthread first because the transaction
kthread wants to wake up the cleaner kthread.  We start the transaction
kthread next because everything in btrfs wants transactions.  We do reloc
recovery in the thread that was doing the original mount call once the
transaction kthread is running.  This means that the cleaner kthread
could already be running when reloc recovery happens (e.g. if a snapshot
delete was started before a crash).

Relocation does not play well with the cleaner kthread, so a mutex was
added in commit 5f316481

 "Btrfs: fix
race between balance recovery and root deletion" to prevent both from
being active at the same time.

If the cleaner kthread is already holding the mutex by the time we get
to btrfs_recover_relocation, the mount will be blocked until at least
one deleted subvolume is cleaned (possibly more if the mount process
doesn't get the lock right away).  During this time (which could be an
arbitrarily long time on a large/slow filesystem), the mount process is
stuck and the filesystem is unnecessarily inaccessible.

Fix this by locking cleaner_mutex before we start cleaner_kthread, and
unlocking the mutex after mount no longer requires it.  This ensures
that the mounting process will not be blocked by the cleaner kthread.
The cleaner kthread is already prepared for mutex contention and will
just go to sleep until the mutex is available.

Signed-off-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>