b=1883

b=1883
r=braam
Wait for dirty data to be handled (writeback, truncate, etc) before issueing
the mdc close.  This is done by tracking opens for write and dirty data and
deferring mdc close at file_release if there aren't other opens for write and
there is dirty data.  We perform these deferred closes when the dirty data is
taken care of or when another open for write comes along.

Also, a hot sanity test to verify the behaviour.