When do_balance() balances the tree, a trick is performed to
provide the ability for other tree writers/readers to check whether
do_balance() is executing concurrently (requires CONFIG_REISERFS_CHECK).

This is done to protect concurrent accesses to the tree. The trick
is the following:

When do_balance is called, a unique global variable called cur_tb
takes a pointer to the current tree to be rebalanced.
Once do_balance finishes its work, cur_tb takes the NULL value.

Then, concurrent tree readers/writers just have to check the value
of cur_tb to ensure do_balance isn't executing concurrently.
If it is, then it proves that schedule() occured on do_balance(),
which then relaxed the bkl that protected the tree.

Now that the bkl has be turned into a mutex, this check is still
fine even though do_balance() becomes preemptible: the write lock
will not be automatically released on schedule(), so the tree is
still protected.

But this is only fine if we have a single reiserfs mountpoint.
Indeed, because the bkl is a global lock, it didn't allowed
concurrent executions between a tree reader/writer in a mount point
and a do_balance() on another tree from another mountpoint.

So assuming all these readers/writers weren't supposed to be
reentrant, the current check now sometimes detect false positives with
the current per-superblock mutex which allows this reentrancy.

This patch keeps the concurrent tree accesses check but moves it
per superblock, so that only trees from a same mount point are
checked to be not accessed concurrently.

[ Impact: fix spurious panic while running several reiserfs mount-points ]

Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>