libxfs: straighten out libxfs_writebuf naming confusion

libxfs: straighten out libxfs_writebuf naming confusion

libxfs_writebuf is not a well named function -- it marks the buffer
dirty and then releases the caller's reference.  The actual write comes
when the cache is flushed, either because someone explicitly told the
cache to flush or because we started buffer reclaim.

Make the buffer release explicit in the callers and rename the function
to say what it actually does -- it marks the buffer dirty outside of
transaction context.

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>