If the swap disk is larger than 1MiB, then use a 1MiB blocksize in `dd`
On my machine using a large block size speeds up swap file creation:
```
/ # time dd if=/dev/zero of=output bs=1024 count=1048576
1048576+0 records in
1048576+0 records out
real 0m 4.61s
user 0m 0.79s
sys 0m 3.77s
/ # time dd if=/dev/zero of=output bs=1048576 count=1024
1024+0 records in
1024+0 records out
real 0m 1.06s
user 0m 0.00s
sys 0m 1.04s
```
Signed-off-by: David Scott <dave.scott@docker.com>
57dd4029c8