zfs_receive_010_pos: change dd arguments

zfs_receive_010_pos: change dd arguments

The  `dd` command as written will not create a hole in the file.
Additionally, the `stride` argument isn't understood by `dd` so
it's replaced with `seek` which isn't equivilant but will result in
a single whole which is sufficient for the test case.  Finally,
`conv=notrunc` is added to avoid truncating the file.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Closes #6023