LU-2139 osc: Fix deadlock bug in brw_commit

This patch fixes a bug in commit:

    commit 07871ab861ba80493c8955fecb8f9cbb598a8b9d
    Author: Prakash Surya <surya1@llnl.gov>
    Date:   Tue Apr 2 13:36:39 2013 -0700

        LU-2139 osc: Track and limit "unstable" pages

which has a race condition that would result in a deadlock scenario. The
fix has also been rolled into patch-set 2 of the upstream Gerrit patch:

    http://review.whamcloud.com/6284

When rebasing onto any future upstream releases, please replace commit
07871ab, with the above Gerrit patch.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: I00183f37ad5fa806a652c6ce6521b36a753c5c6c