f2fs: fix to check upper boundary for gc_valid_thresh_ratio

This patch adds missing upper boundary check while setting
gc_valid_thresh_ratio via sysfs.

Fixes: e791d00b ("f2fs: add valid block ratio not to do excessive GC for one time GC")
Cc: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>