A user reported a deadlock where the async submit thread was blocked on the
lock_extent() lock, and then everybody behind him was locked on the page lock
for the page he was holding.  Looking at the code I noticed we do not unlock the
extent range when we get ENOSPC and goto retry.  This is bad because we
immediately try to lock that range again to do the cow, which will cause a
deadlock.  Fix this by unlocking the range.  Thanks,
Signed-off-by: Josef Bacik <jbacik@fusionio.com>