libxfs: fix buffer refcounting in delwri_queue

libxfs: fix buffer refcounting in delwri_queue

In the kernel, xfs_buf_delwri_queue increments the buffer reference
count before putting the buffer on the buffer list, and the refcount is
decremented after the io completes for a net refcount change of zero.

In userspace, delwri_queue calls libxfs_writebuf, which puts the buffer.
delwri_queue is a no-op, for a net refcount change of -1.  This creates
problems for any callers that expect a net change of zero, so increment
the buffer refcount before calling writebuf.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>