btrfs: Add WARN_ON() for double lock in btrfs_tree_lock()

When a task trying to double lock a extent buffer, there are no
lockdep warning about it because this lock may be in "blocking_lock"
state, and make us hard to debug.

This patch add a WARN_ON() for above condition, it can not report
all deadlock cases(as lock between tasks), but at least helps us
some.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>