ceph: fix ceph_dir_llseek()

Comparing offset with inode->i_sb->s_maxbytes doesn't make sense for
directory. For a fragmented directory, offset (frag_t, off) can be
larger than inode->i_sb->s_maxbytes.

ceph_dir_llseek() initializes variable 'old_offset' to the value of
parameter 'offset'. This doesn't make sense neither. 'old_offset'
should be ceph_make_fpos(fi->frag, fi->next_offset).

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>