gfs2: move setting current->backing_dev_info

Set current->backing_dev_info just around the buffered write calls to
prepare for the next fix.

Fixes: 967bcc91 ("gfs2: iomap direct I/O support")
Cc: stable@vger.kernel.org # v4.19+
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>