add missing printk loglevel in mm/swapfile.c

in mm/swapfile.c a printk() is missing a loglevel. I believe the proper
loglevel for this situation is KERN_ERR, so that's what the patch below
sets -if you agree, please apply.

Signed-off-by: Jesper Juhl <juhl-lkml@dif.dk>
Signed-off-by: Adrian Bunk <bunk@stusta.de>