f2fs: fix to limit gc_pin_file_threshold

type of f2fs_inode.i_gc_failures, f2fs_inode_info.i_gc_failures, and
f2fs_sb_info.gc_pin_file_threshold is __le16, unsigned int, and u64,
so it will cause truncation during comparison and persistence.

Unifying variable of these three variables to unsigned short, and
add an upper boundary limitation for gc_pin_file_threshold.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>