Linux 4.9 compat: remove iops->{set,get,remove}xattr

In Linux 4.9, torvalds/linux@fd50eca, iops->{set,get,remove}xattr and
generic_{set,get,remove}xattr are removed. xattr operations will directly
go through sb->s_xattr.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>