Repair bufmgr deadlock problem reported by Michael Wildpaner. Must take

Repair bufmgr deadlock problem reported by Michael Wildpaner.  Must take
share lock on a buffer being written out before releasing BufMgrLock in
the BufferAlloc code path; if we do it later we might block on someone
who's re-pinned the buffer.  I believe this is only an issue for BufferAlloc
and not the other places that call FlushBuffer.  BufferSync must continue
to do it the old way since it may well be trying to write buffers that
other backends have pinned; but it should not be holding any conflicting
locks.  FlushRelationBuffers is okay since it's got exclusive lock at the
relation level.