ceph: improving efficiency of syncfs

write_inode() could be called variety of reasons, in the case of syncfs(2)
there is no need to wait for flush getting completed in write_inode(),
->sync_fs is for guaranteeing flush completion for all inodes at that point.

Signed-off-by: Chengguang Xu <cgxu519@icloud.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>