iomap: fix short copy in iomap_write_iter()

Starting with commit 5d8edfb9 ("iomap: Copy larger chunks from
userspace"), iomap_write_iter() can get into endless loop. This can
be reproduced with LTP writev07 which uses partially valid iovecs:
        struct iovec wr_iovec[] = {
                { buffer, 64 },
                { bad_addr, 64 },
                { buffer + 64, 64 },
                { buffer + 64 * 2, 64 },
        };

commit bc1bb416 ("generic_perform_write()/iomap_write_actor():
saner logics for short copy") previously introduced the logic, which
made short copy retry in next iteration with amount of "bytes" it
managed to copy:

                if (unlikely(status == 0)) {
                        /*
                         * A short copy made iomap_write_end() reject the
                         * thing entirely.  Might be memory poisoning
                         * halfway through, might be a race with munmap,
                         * might be severe memory pressure.
                         */
                        if (copied)
                                bytes = copied;

However, since 5d8edfb9 "bytes" is no longer carried into next
iteration, because it is now always initialized at the beginning of
the loop. And for iov_iter_count < PAGE_SIZE, "bytes" ends up with
same value as previous iteration, making the loop retry same copy
over and over, which leads to writev07 testcase hanging.

Make next iteration retry with amount of bytes we managed to copy.

Fixes: 5d8edfb9

 ("iomap: Copy larger chunks from userspace")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>