iov_iter_bvec() initialises iterators well, no need to pre-zero it
beforehand as done in fd_execute_rw_aio(). Compilers can't optimise it out
and generate extra code for that (confirmed with assembly).

Link: https://lore.kernel.org/r/34cd22d6cec046e3adf402accb1453cc255b9042.1610207523.git.asml.silence@gmail.comReviewed-by: NBart Van Assche <bvanassche@acm.org>
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>