ceph: Correctly return NXIO errors from ceph_llseek

ceph_llseek does not correctly return NXIO errors because the 'out' path
always returns 'offset'.

Fixes: 06222e49 ("fs: handle SEEK_HOLE/SEEK_DATA properly in all fs's that define their own llseek")
Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
Signed-off-by: Yan, Zheng <zyan@redhat.com>