f2fs: export ipu policy in debugfs

Export ipu_policy as a string in debugfs for better readability and
it can help us better understand some strategies of the file system.

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