drivers/block/loop.c: ratelimit error messages

drivers/block/loop.c: ratelimit error messages

Metric tons of high speed spew is not helpful when things go pear shaped.
systemd lost its mind, forgot how to stop services it insists on being
sole manager of, massive printk() flood ensued, box eventually died.

[16206.684000] loop: Write error at byte offset 11412291584, length 4096.
[16206.684000] systemd-journald[1758]: /dev/kmsg buffer overrun, some messages lost.
[16206.684000] loop: Write error at byte offset 13155434496, length 4096.
[16206.684000] loop: Write error at byte offset 13155438592, length 4096.
[16206.684000] loop: Write error at byte offset 13155442688, length 4096.
[16206.684000] loop: Write error at byte offset 13960736768, length 4096.
[16206.684000] loop: Write error at byte offset 14229172224, length 4096.
[16206.684000] systemd-journald[1758]: /dev/kmsg buffer overrun, some messages lost.
[16206.684000] loop: Write error at byte offset 14766043136, length 4096.
[16206.684000] loop: Write error at byte offset 15034478592, length 4096.
[16206.684000] systemd-journald[1758]: /dev/kmsg buffer overrun, some messages lost.

Signed-off-by: Mike Galbraith <bitbucket@online.de>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@fb.com>