libxfs: clear the buffer error while the buffer is locked

libxfs: clear the buffer error while the buffer is locked

When releasing a buffer, the error shoul dbe cleared while the lock
is still held on the buffer to avoid racing with a new user of the
buffer.

This was pointed out in review of commit 6af7c1e ("libxfs: reused
invalidated buffers leak state and data") but the version committed
didn't have the fix. Thanks to Christoph Hellwig for checking and
pointing out the oversight.

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>