LU-7343 osd-ldiskfs: handle ldiskfs_append failure

LU-7343 osd-ldiskfs: handle ldiskfs_append failure

In new linux kernel (linux-3.1x, x>=0), the ldiskfs exported
function ldiskfs_append() return error# via the return value,
instead of via the output parameter @err as it does on other
kernels (linux-2.6). Under such case, the caller should not
assume non-NULL returned value is valid buffer head, it can
stands for error#. So check that properly.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I4dca43bcfd31aafd999f54934a51d258071dab22
Reviewed-on: http://review.whamcloud.com/17148
Tested-by: Jenkins
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>