Prevent deadlock in io-stats list management of top.
Prevent deadlock in io-stats list management of top.

Senario: thread t1 has lock on the queue, and waits on the element lock to remove
it and unref it. Thread t2 has a lock on the element being removed from the queue,
but waiting on the queue on the list to update it.

The fix is to move list management to outside of the lock. Additonal to this change
is adding locks to ios_stat_ref

Signed-off-by: shishir gowda <shishirng@gluster.com>
Signed-off-by: Vijay Bellur <vijay@dev.gluster.com>

BUG: 2578 (Mountpoint hung while running the dbench)
URL: http://bugs.gluster.com/cgi-bin/bugzilla3/show_bug.cgi?id=2578

Kaushikbv/Gluster,Kaushikbv/Gluster,Kaushikbv/Gluster