Remove iov_iter_advance() for iter_write

The additional iter advance is incorrect, as copy_from_iter() has
already done the right thing. This will result in the following
warning being printed to the console as of the 5.12 kernel.

    Attempted to advance past end of bvec iter

This change should have been included with #11378 when a
similar change was made on the read side.

Suggested-by: @siebenmann
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Issue #11378
Closes #12041
Closes #12155