lustre/osc: Don't flush active extents.

The extent is active so we need to abort and let the caller
re-dirty the page. If we continued on here, and we were the
one making the extent active, we could deadlock waiting for
the page writeback to clear but it won't because the extent
is active and won't be written out.

Signed-off-by: Ann Koehler <amk@cray.com>
Reviewed-on: http://review.whamcloud.com/8278
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4253


Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Alexey Lyashkov <alexey_lyashkov@xyratex.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>