ceph: check negative offsets in ceph_llseek()

When a user requests SEEK_HOLE or SEEK_DATA with a negative offset
ceph_llseek should return -ENXIO.  Currently -EINVAL is being returned for
SEEK_DATA and 0 for SEEK_HOLE.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>