bcache: fix input integer overflow of congested threshold

Cache set congested threshold values congested_read_threshold_us and
congested_write_threshold_us can be set via sysfs interface. These
two values are 'unsigned int' type, but sysfs interface uses strtoul
to convert input string. So if people input a large number like
9999999999, the value indeed set is 1410065407, which is not expected
behavior.

This patch replaces sysfs_strtoul() by sysfs_strtoul_clamp() when
convert input string to unsigned int value, and set value range in
[0, UINT_MAX], to avoid the above integer overflow errors.

Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>