block: Always check length of all iov entries in blk_rq_map_user_iov()

Even after commit 54787556
("block: check for proper length of iov entries earlier ...")
we still won't check for zero-length entries after an unaligned
entry.  Remove the break-statement, so all entries are checked.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>