- Don't acquire the vnode interlock in drain_output(). Instead, require the

   caller to acquire it.  This permits drain_output() to be done atomically
   with other operations as well as reducing the number of lock operations.
 - Assert that the proper locks are held in drain_output().
 - Change getdirtybuf() to accept a mutex as an argument.  This mutex is used
   to protect the vnode's buf list and the BKGRDWAIT flag.  This lock is
   dropped when we successfully acquire a buffer and held on return
   otherwise.  These semantics reduce the number of cumbersome cases in
   calling code.
 - Pass the mtx from getdirtybuf() into interlocked_sleep() and allow this
   mutex to be used as the interlock argument to BUF_LOCK() in the LOCKBUF
   case of interlocked_sleep().
 - Change the return value of getdirtybuf() to be the resulting locked buffer
   or NULL otherwise.  This is for callers who pass in a list head that
   requires a lock.  It is necessary since the lock that protects the list
   head must be dropped in getdirtybuf() so that we don't have a lock order
   reversal with the buf queues lock in bremfree().
 - Adjust all callers of getdirtybuf() to match the new semantics.
 - Add a comment in indir_trunc() that points at unlocked access to a buf.
   This may also be one of the last instances of incore() in the tree.

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