f2fs: use seq_puts()/seq_putc() rather than seq_printf() where possible

For string without format specifiers, using seq_puts()/seq_putc()
instead of seq_printf().

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>