ceph: convert to generic_file_llseek

ceph: convert to generic_file_llseek

There's no reason we need to lock the inode for write in order to handle
an llseek. I suspect this should have been dropped in 2013 when we
stopped doing vmtruncate in llseek.

With that gone, ceph_llseek is mostly equivalent to generic_file_llseek,
so just call that after getting the size.

Signed-off-by: Jeff Layton <jlayton@kernel.org>