bdev: fix write zeroes fallback error path

If an I/O fails during the write zeroes fallback to write on a split
I/O, the previous code would call spdk_bdev_io_complete() again on the
same I/O, causing infinite recursion.

Fix this by restoring the original user callback before completing the
I/O, as well as using _spdk_bdev_io_complete() rather than
spdk_bdev_io_complete() to avoid double-counting the statistics.

This also preserves the original bdev_io->status code set by the module,
rather than always setting it to SPDK_BDEV_IO_STATUS_FAILED.

Change-Id: Ib71fa6e54eed3d5823652971f7c439e0ff236fc1
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>
Reviewed-on: https://review.gerrithub.io/383506


Reviewed-by: Seth Howell <seth.howell5141@gmail.com>
Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Ziye Yang <optimistyzy@gmail.com>