ceph: remove sb_start/end_write in ceph_aio_write.

Either in vfs_write or io_submit,it call file_start/end_write.
The different between file_start/end_write and sb_start/end_write is
file_ only handle regular file.But i think in ceph_aio_write,it only
for regular file.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Acked-by: Yan, Zheng <zheng.z.yan@intel.com>
possible btrfs fix from chris