zfs: Fix build breakage in zfs_setattr()

OSv does not support xattrs in ZFS. Fix up a build breakage reported by Glauber:

  /home/ubuntu/osv/bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c: In function ‘zfs_setattr’:
  /home/ubuntu/osv/bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c:2152:12: error: ‘xoap’ may be used uninitialized in this function [-Werror=uninitialized]

Reported-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>