backing-dev: use kstrto* in preference to simple_strtoul

Fix checkpatch warnings:

WARNING: consider using kstrto* in preference to simple_strtoul

for the below sys entry parsers:
/sys/block/<block device>/bdi/read_ahead_kb
/sys/block/<block device>/bdi/max_ratio
/sys/block/<block device>/bdi/min_ratio

Signed-off-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Vivek Trivedi <vtrivedi018@gmail.com>