f2fs: improve sbi status info in debugfs/f2fs/status

Do not use numbers but strings to improve readability when flag is set.

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