backing-dev: use kstrto* in preference to simple_strtoul
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 <4eaf47c25ead9675aad93689a157e819ffdaf8b3@gmail.com>
Signed-off-by: Vivek Trivedi <8d27efd872215d9b261cee2bcbb54eeda6647549@gmail.com>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs