Remove zfs_getattr and convoff dead code

Remove zfs_getattr and convoff dead code

The `convoff` function is called only in one code path in `zfs_space`.
Each caller of `zfs_space` is called with a `flock64_t` that has
`l_whence` set to `SEEK_SET`. This means that `convoff` always results
in a no-op as the `bfp` parameter has `l_whence` set to `SEEK_SET` and
`int whence` is `SEEK_SET` as well.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by:  Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Dirkjan Bussink <d.bussink@gmail.com>
Closes #10006