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

include/sys/dmu.h

@@ -938,7 +938,7 @@ void dmu_object_info_from_dnode(dnode_t *dn, dmu_object_info_t *doi);
 void dmu_object_info_from_db(dmu_buf_t *db, dmu_object_info_t *doi);
 /*
  * Like dmu_object_info_from_db, but faster still when you only care about
- * the size.  This is specifically optimized for zfs_getattr().
+ * the size.
  */
 void dmu_object_size_from_db(dmu_buf_t *db, uint32_t *blksize,
     u_longlong_t *nblk512);