f2fs: convert to MAX_SBI_FLAG instead of 32 in stat_show()

BIW reduce the s_flag array size and make s_flag constant.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>